@David-TP 

I installed this firmware but I notice that my WiFi network is much less stable than before. Regularly one of the sub decos cannot find the main deco, then the sub blinks red. And the main deco sometimes has a solid red. After about a minute the solid white LED is back but this behaviour did not occur with the 1.5.8 firmware.

Also troublesome: I regularly cannot reach the Deco's via the app. Get the message "We could not find Deco" even when the LEDs of all Deco's are solid white, I can connect to the Deco's WiFi and internet connection is available. Have rebooted the Decos several times (manually, cut of the power) without success.

Because I had so many disturbances I just temporarily put my three old access points back in place. Can try flashing back the old firmware but thought it would be better to do some troubleshooting with the beta firmware first.

I had connected the Deco's as follows:
Modem/router ISP > TP-Link LS1005G switch > 3 Deco M4s. The Deco's are in accespoint mode.

@David-TP 

Yes, al three units are connected to the switch via ethernet cables. The red LED on the Deco occurs about twice an hour so yes, I can reproduce it.
I've created a test setup this mornig, put the 3 M4s together and connected them in the same way as they were on the 1st, 2nd and third floors (switch, ethernet) only much closer to each other.

I was planning to create a log this way but after an hour or so I noticed that I lost internet connection completely through my temporary former Arcadyan access points. These Arcadyan's were also connected through a switch, with LAN cables. The two slave Deco's had a flashing red light, the main one has a solid red light. I disconnected them from power after this cause I need internet for my work, then I had wifi through my Arcadyan's again. Very strange.

Tomorrow evening I'll test again, I hope I can provide some logs. I wasn't on the latest Deco app, now I am so for the test tomorrow I will use the upated one.

  @David-TP I have 2 M4s running v1.5.8 in AP mode. Is there any benefit to me upgrading to this version?

How many of these features are available in AP mode V Route mode?

Cheers.

Upgraded just now, I have two Decos M4 connected via ethernet and one M3W unit. The main reason was disabling Smart DHCP. Thanks a lot for the upgrade! Currently, I do not have any issues. If anything pops out, I will write here.

Just a small question - I suspect the source selection does not work for me because of the M3W unit....? Is there a plan to support that unit too?

Many thanks again!

First update. There is indeed something wrong with the firmware. I have two m4 units and a single m3w unit. The system is set up in AP mode and Smart DHCP is off. The Deco m4 units are connected through Ethernet (there is one TP-Link unmanaged switch between them). For m3w, I am not sure to which unit it connects, probably the main deco. Note that this information is not available for m3w in the app.

The problem I experience is that approximately three times an hour the network just fails and even the devices with static IP using Ethernet get inaccessible for a short time. Somehow this suggests some kind of a packet storm in the network caused by the decos. I sent the app log just now.

EDIT: at the time of the network issues, the slave unit is flashing red. I am now in the process of going back to 1.5.8.

EDIT2: when going back to 1.5.8, I first downgraded the slave unit. However, it failed to connect to the master deco via Ethernet, only wifi backhaul. So I removed the slave device from the network and did its factory reset. Still, it was not possible to add it to the old network! Finally, I downgraded the master unit and everything worked from then. My conclusion: it must be something related to bad communication of the master unit, and this is probably worse with Ethernet backhaul 

EDIT3: the network drops were occuring even when having a single M4 unit (master)

@David-TP This morning at 10:33AM I sent a logfile. I recieved two confirmation mails so I think you recieved two.

This is what happened: The main Deco of course had a yellow LED at first after booting up, then static white but after only 5 minutes it turned red. After a while it turned white again, but after about 10 minutes it turned red again. After about 2 hours, my former network was completely disrupted again and Wifi via the old access points was no longer possible

What I was wondering: how far does such a log file go back? The Decos were offline until this morning, does the log start with the time of booting?

@Bugs22 In AP mode the following from the list is not available: Support for NO-IP and DynDNS and Support for setting Effective Time and Bandwidth Limit for guest network. Add Static Route and the ability to customize the MTU is also not available in my opinion.