Many homes end up with two routers: the box the provider supplied, plus one bought later for better coverage. Each one hands out its own private addresses, so devices end up on two separate networks that cannot see each other.
The symptoms are distinctive. The internet works fine, but a phone cannot find the printer, the TV cannot see the media server, and a camera app fails whenever you are at home even though it works on mobile data.
It also breaks anything that needs an incoming connection: game consoles report a strict network type, remote access to a home server fails, and video calls fall back to slower relay servers.
The fix is to have exactly one device doing the routing. Either put the provider box into modem or bridge mode, or set the second router to access-point mode so it only extends the Wi-Fi.