Question about Linksys Velop Tri Band

Basically, the interior walls are purely drywall, i.e. no concrete, etc. The provider's router is at end A of the flat and now the problem is that the two rooms in end B, i.e. in the exact opposite direction, hardly have any WiFi.

You can't say that as a general rule. Basically, I would have said that a good Wi-Fi router is enough.
How is the signal strength now with a WLAN router? Are there any "dead spots" in the WLAN connection?
Can you place the routers strategically so that the whole flat is covered 50/50?
How are the walls built?