@TP-Link 

Devices which are connected to other Deco "main, attic, living room" are reachable from devices connected to the "upstairs" Deco. Only the devices which are connected to the "garage" are not reachable from the clients connected to "upstairs" Deco using Ethernet backhaul.

Switch type: TL-SG1008D. It's an unmanaged switch, there is no way to configure VLANs on it.

There is an Intel NUC connected to this switch, from this device I am able to ping any devices connected to the "garage" Deco, or any device connected to the "upstairs" Deco, also the Decos itself are pingable from this source.

Internet access is also available on all devices connected to whichever Deco, only local LAN issue is visible as I described.

@TP-Link,

is there any news here? Since then I have installed the latest 1.5.3 version on the M5 Decos, but the problem is still not being resolved. 

@TP-Link,

any news?

I could manage to have a new setup: ISP router --- Main Deco M5 --- TP-Link switch. From the switch, the following Decos are connected with Ethernet backhaul: Garage M5, Upstairs M4R. Two Decos are still connected with Wireless backhaul: Living Room M5 and Attic M4R.

Some clients connected to the Garage M5 are still not available when I am in the near of Living Room or Attic. Running a ping test, when I'm continuously pinging the IP of a device which is wirelessly connected to Garage M5, if I go to the near of living room or attic, the clients are not responding to ping. If I go back to other decos which are Ethernet backhaul connected, ping works again. Going to Wireless backhaul connected deco again, ping has got timeot.

Problematic devices can go to the WAN/Internet: for example one problematic device is an IP camera connected to Apple Homekit Secure video, recordings are available all the time, but live view is not available if I want to check it in the near from wireless backhaul connected decos. It's weird.

@TP-Link,

Today I did further tests, with a Powerline adapter I was able to connect the living room M5 also with Ethernet backhaul to the main deco via the switch. Once the connection is established the previously not responding devices are suddenly responding to ping near to the living room deco too. When I remove then the ethernet cable the ping test is still successfull. 

Unfortunately the powerline adapter is not the best solution due to poor bandwidth (only 40-50 Mbit/s) so I removed it and went back to wireless backhaul. After removal of the cable the ping tests and connections are fine.

As further test I rebooted the living room deco, then the original problem came back. Then again with powerline and ethernet backhaul the problem is solved, removed the cable all good again, until the next reboot. 

Please help to solve this weird problem!!!

@TP-Link 

checking the logs after today's test it's clearly obvious, ARP table of the "living room" does not have the client entries connected to another deco. The log entries are from the same time when I did the test walking around the house during the ping test. The ARP error issues pop up, then disappear and pop-up again, when I was near the living room deco:

ed Nov  3 15:02:12 2021 daemon.err client_mgmt: client(40-A3-CC-1F-54-28) not found in arp table disconnect_counter=0
Wed Nov  3 15:02:12 2021 daemon.err client_mgmt: client(94-B9-7E-E0-3E-24) not found in arp table disconnect_counter=0
Wed Nov  3 15:02:16 2021 daemon.err udhcpd[6280]: Sending ACK to 192.168.68.139
Wed Nov  3 15:02:16 2021 daemon.err udhcpd[6280]: clear ip 8b44a8c0
Wed Nov  3 15:02:32 2021 daemon.err client_mgmt: client(40-A3-CC-1F-54-28) not found in arp table disconnect_counter=1
Wed Nov  3 15:02:41 2021 user.warn IMPROXY: WARNING[mcast_recv_igmp@753]: Don't exist this Downstream VIF
Wed Nov  3 15:02:53 2021 daemon.err client_mgmt: client(40-A3-CC-1F-54-28) not found in arp table disconnect_counter=2
Wed Nov  3 15:02:53 2021 daemon.err udhcpd[6280]: Found static lease: 6b44a8c0
Wed Nov  3 15:02:53 2021 daemon.err udhcpd[6280]: Sending ACK to 192.168.68.107
Wed Nov  3 15:02:53 2021 daemon.err udhcpd[6280]: clear ip 6b44a8c0
Wed Nov  3 15:03:09 2021 daemon.err udhcpd[6280]: Sending ACK to 192.168.68.106
Wed Nov  3 15:03:09 2021 daemon.err udhcpd[6280]: clear ip 6a44a8c0
Wed Nov  3 15:03:35 2021 daemon.err client_mgmt: client(44-73-D6-0D-E2-6C) not found in arp table disconnect_counter=0
Wed Nov  3 15:03:56 2021 daemon.err client_mgmt: client(44-73-D6-0D-E2-6C) not found in arp table disconnect_counter=1
Wed Nov  3 15:04:15 2021 daemon.err client_mgmt: client(3C-71-BF-20-83-D2) not found in arp table disconnect_counter=0
Wed Nov  3 15:04:15 2021 daemon.err client_mgmt: client(44-73-D6-0D-E2-6C) not found in arp table disconnect_counter=2
Wed Nov  3 15:04:15 2021 daemon.err client_mgmt: client(D0-3F-AA-5C-E9-AC) not found in arp table disconnect_counter=0
Wed Nov  3 15:04:36 2021 daemon.err client_mgmt: client(40-A3-CC-1F-54-28) not found in arp table disconnect_counter=0
Wed Nov  3 15:04:37 2021 daemon.err udhcpd[6280]: Found static lease: 7444a8c0
Wed Nov  3 15:04:37 2021 daemon.err udhcpd[6280]: Sending ACK to 192.168.68.116
Wed Nov  3 15:04:38 2021 daemon.err udhcpd[6280]: clear ip 7444a8c0
Wed Nov  3 15:04:40 2021 daemon.err udhcpd[6280]: Sending ACK to 192.168.68.125
Wed Nov  3 15:04:40 2021 daemon.err udhcpd[6280]: clear ip 7d44a8c0
Wed Nov  3 15:04:46 2021 user.warn IMPROXY: WARNING[mcast_recv_igmp@753]: Don't exist this Downstream VIF
Wed Nov  3 15:04:56 2021 daemon.err client_mgmt: client(40-A3-CC-1F-54-28) not found in arp table disconnect_counter=1
Wed Nov  3 15:04:56 2021 daemon.err client_mgmt: client(34-7E-5C-37-99-6C) not found in arp table disconnect_counter=0
Wed Nov  3 15:05:16 2021 daemon.err client_mgmt: client(40-A3-CC-1F-54-28) not found in arp table disconnect_counter=2
Wed Nov  3 15:05:17 2021 daemon.err client_mgmt: client(70-A2-B3-E2-E6-1F) not found in arp table disconnect_counter=0
Wed Nov  3 15:05:17 2021 daemon.err client_mgmt: client(34-7E-5C-37-99-6C) not found in arp table disconnect_counter=1
Wed Nov  3 15:05:59 2021 daemon.err udhcpd[6280]: Sending ACK to 192.168.68.151
Wed Nov  3 15:05:59 2021 daemon.err udhcpd[6280]: clear ip 9744a8c0
Wed Nov  3 15:06:43 2021 daemon.err client_mgmt: wireless client associate or disassociate.
Wed Nov  3 15:06:43 2021 daemon.err client_mgmt: client_mac:D0-3F-AA-5C-E9-AC, client_ifname:ath1, client_action:associate
Wed Nov  3 15:06:43 2021 daemon.err client_mgmt: client(D0-3F-AA-5C-E9-AC) assocaite by if(ath1).
Wed Nov  3 15:06:51 2021 user.warn IMPROXY: WARNING[mcast_recv_igmp@753]: Don't exist this Downstream VIF
Wed Nov  3 15:06:57 2021 daemon.err client_mgmt: client(58-D3-49-2D-D2-57) not found in arp table disconnect_counter=0
Wed Nov  3 15:07:06 2021 daemon.err client_mgmt: wireless client associate or disassociate.
Wed Nov  3 15:07:06 2021 daemon.err client_mgmt: client_mac:D0-3F-AA-5C-E9-AC, client_ifname:ath1, client_action:disassociate
Wed Nov  3 15:07:06 2021 daemon.err client_mgmt: client(D0-3F-AA-5C-E9-AC) disassocaite by if(ath1).
Wed Nov  3 15:07:17 2021 daemon.err client_mgmt: client(58-D3-49-2D-D2-57) not found in arp table disconnect_counter=1
Wed Nov  3 15:07:17 2021 daemon.err client_mgmt: client(78-28-CA-84-E0-0E) not found in arp table disconnect_counter=0
Wed Nov  3 15:07:36 2021 daemon.err client_mgmt: client(58-D3-49-2D-D2-57) not found in arp table disconnect_counter=2
Wed Nov  3 15:08:21 2021 authpriv.warn dropbear[20205]: Login attempt for nonexistent user from 192.168.68.139:59215
Wed Nov  3 15:08:37 2021 daemon.err client_mgmt: client(58-D3-49-10-7E-3C) not found in arp table disconnect_counter=0
Wed Nov  3 15:08:56 2021 user.warn IMPROXY: WARNING[mcast_recv_igmp@753]: Don't exist this Downstream VIF
Wed Nov  3 15:08:57 2021 daemon.err client_mgmt: client(3C-71-BF-20-93-5C) not found in arp table disconnect_counter=0
Wed Nov  3 15:08:57 2021 daemon.err client_mgmt: client(58-D3-49-10-7E-3C) not found in arp table disconnect_counter=1

The MAC addresses 40-A3-CC-1F-54-28, 44-73-D6-0D-E2-6C, 3C-71-BF-20-83-D2 belong to the devices which are connected to the "garage deco". So: ethernet backhaul decos and wireless connected decos does not share/keep sync their ARP tables! 

If all decos are Wireless backhaul connected or ALL decos are Ethernet backhaul connected, the issue is not visible. If the backhauls are mixed, seems to be the ARP table is not synced correctly. Please check!