Question about TP-Link Deco M4 AC1200

Yes, that is possible, but you have to bear in mind that with a mesh system, the WLAN of the Internet router is switched off and the mesh router then takes over all network traffic. Another interesting variant would be the Deco M5 solution, which even has an integrated and free antivirus and antimalware solution, which is free of charge (HomeCare).

TP-Link Deco M5 Dualband WLAN Mesh 3er Set (AC1300)

Or alternatively, there is the Xiaomi AX3000 system, which even includes a 3 port Gbit switch per mesh router and is WiFi 6 capable for up to 256 devices:

Xiaomi Mesh System AX3000 (2-Pack)

This is like a normal wireless router and works with any wireless-enabled device.

Yes, it works very well with the UPC Box. Of course, you should switch off the WLAN on the UPC Box, but you probably know that.