In my opinion, you can't give a blanket answer to that. Factor 1: How good is the mobile reception at home? Is a good 4G connection possible with the chosen provider? Factor 2: How good is the WLAN coverage in the flat? Depending on the building construction, it can be better or worse. It is best to test the speed with a smartphone hotspot.

If you want an LTE router with higher coverage, I recommend buying a router with external antennas. It can have better coverage.
For example, Archer MR600
Tp-Link Archer MR600

Hello. Yes, this is possible. Only the maximum number of connected devices is limited. Unfortunately, I can't find the number right now. The ping for online gaming will be rather poor with the router. Since 4G routers are generally not the best here. If you have 20Mbit via ADSL, this would be preferable for online gaming. Is your question answered?