@Yoji 

You can ignore "IP address missing in Deco app" issue, it is not related to printer connectivity. I have Deco M5 mesh running in AP mode and TP-Link security camera that never shows its IP address in Deco app. I know camera is all right, because it streams 24/7 to my NAS. No interruptions. It, too, has static IP address set on router. 

For now, you can ignore that part. This seem to be Deco app reporting bug.

Your printer does not have Ethernet port unfortunately, using it could have been a workaround. Printer is relatively old, as far as I can tell, which means it could have issues with either WiFi mesh protocols or dual-band Main Network (5GHz and 2.4GHz under same SSID) if you run it that way, which is by default.

With Deco M5 you can enable IoT Network which by default only runs on 2.4GHz. I suggest you do that, have that IoT Network with its own unique SSID, connect printer to IoT Network SSID.

If printer can't handle dual band, this will solve the problem. If not, there must be issues with printer handling WiFi mesh protocols (multiple nodes broadcasting same SSID and telling device which node to connect to). There are few options to try to deal with that, but first you should set IoT Network and hopefully that will be enough.

  @Alexandre. 

Thanks,  via the App I have disabled the "Mesh Technology" option for the printer.  I see I can see from the "Connection preference" options I can limit its connection to a specific Deco, and also to a specific frequency (but I have not done this yet, it confirms it is connecting via 2.4 GHz at the moment and working).
I will see how I get on with Mesh disabled.. and fix the Deco and frequency if I continue to have problems.. and try your whole new IoT SSID only if I have to.
Thanks again for your feedback, it gives me a good path to resolution (hopefully).