WiFi is poor in the house despite a repeater, what to do?

smells up to the surrounding neighbors.

Oh dear ...

LOL well, just two swapped letters already result in a completely different meaning

I have a 2nd router on the other floor that has the same network name. So you connect to the same network on different floors.

God gave us MESH for that, but only the expensive routers can do that

Router only works per floor, even the expensive Fritz ones. Therefore mesh or additionally an access point on each floor.

I did not like the APs on the ceiling. Therefore, we will install the TP-Link EAP 225 Wall. The network socket will be placed just below the ceiling. It is also only 1.5 cm thick and therefore quite inconspicuous.

How and where the access points are is relatively irrelevant (without wanting to start a fundamental discussion here). If the concrete ceilings etc. cause interference, then one access point per floor...on the wall or ceiling or on the shelf...done.