I was havng the exact same issue and stumbled across this thread.  I changed the Deco to access point mode and everything now works the way i want it.  Thanks for the info.

I have the same problem. My TP-Link router with connected NAS and USB device worked fine, both devices could be mapped via the router's WiFi. Then I bought a Deco M4 for the WiFi, disabled the router's WiFi, and now I cannot access the connected devices when I use the Deco's WiFi. With a direct LAN connection to the router everything works ok (as before). I have tried both "Access Point" and "Router" setting for the Deco, neither works. Any Ideas?

In my desperation I just tried something crazy: I disconnected the NAS from the router, took it to the basement next to the Deco M4 #3 and connected Deco and NAS with a LAN cable. Then I took my smartphone, checked the network neighbourhood (where before there was nothing to be found) and as if by magic there was the NAS. And it was not alone, I also saw the USB device connected directly to the router. Then I checked my desktop PC, which does not use WiFi but connects directly to the router via LAN, and there was the NAS, and the USB device. So now everything works but I don't know why it didn't before.