Classic misunderstanding with mesh. The mesh network builds up its own IP range. All devices must be unplugged from the main router and connected to the mesh router system, otherwise the devices will not see each other because they are in different subnets.

The best thing to do now is to buy a small 5-pin switch and connect it to a Deco where you need more than 2 connections. The router of the internet provider only serves as a quasi internet modem in a classic mesh setup. The WLAN of the Internet service provider router should also be switched off, as it is no longer needed and the mesh network has taken over.