My main router is a Deco P9 and I am using a RE505X as a "client mode" 

With client mode, it can connect to a wired device and works as a wireless adapter to receive wireless signal from your wireless network.

I am just using the 5 GHZ band to connect to the Deco P9 Wifi and I disable the extended network. Also the RE505X obtains an IP address automatically, the DHCP server is off as well as the access control. Everything is handle by the Deco P9.

I have a switch connected to the RE505X Ethernet port and all the devices connected to the switch have internet access but if i want to connect to the devices that are behind the RE505X from the Deco P9 Wifi it is not possible.

I also used an Archer 80 router in wireless distribution system (WDS) mode, but it happens the same.

Finally,  instead of using the RE505X, temporally I place another P9 with the switch and I was able to connect, but i don't want to use another Deco (I don't need the wifi and it can be turn off)