My parents have a Spark Smart Modem 3 doing routing and DHCP, and a pair of Deco X20s in AP mode (one with ethernet back to the SM3, the second wireless mesh to the first).

The Decos and the SM3 are both set up with the same SSID and password, so devices 'roam' between all three fine, and there are no issues with internet access.

However, it appears (I can't confirm 100% myself, but I am told) that there are issues when trying to communicate within the network:

• The printer is connected via ethernet to the SM3. When client devices are connected to wifi from a Deco, printing doesn't appear to work. It does work when connected to wifi from the SM3.
• Streaming via Airplay to the soundbar (which is via wifi to the SM3) appears not to work when connected to wifi via the Deco. Again, it works when connected to wifi via the SM3.

I'm not a networking guy, but I assume that there is something preventing or hindering LAN traffic here? Would someone be able to explain this in simple terms? Is it expected behaviour that devices on the two different 'sides' of this network setup can't communicate with each other, even though they are getting IPs within the same range from the same gateway?

I assume that the problem will go away if I purchase a third Deco to replace the SM3.

However, before I do, I wanted to check if this behaviour was expected, and if there was any device or network config setting that could fix this issue.