Multiplayer ports blocked?

I hang with my router behind the router from the residents in the apartment below me (why is unimportant, can't be changed) and the Wlan / Lan prevents all possible multiplayer actions, I can neither chat with my friends via Discord (but writing is possible ) still play Minecraft online or play online with my switch with my friends.

Otherwise, the internet connection runs perfectly on all devices and even when I try it over the LAN it doesn't work…

I probably have limited guest access, is there a specific port that needs to be shared?

Or what can I do to play online? (In principle it is not my router)

If this is a guest access via a Fritzbox, then you (your router) have the guest profile with them. You would either have to change the profile or give the guest unlimited rights.

You know yourself that the solution is actually completely stupid. You could clip a third router to the connector and then connect your two routers behind it. This would completely separate the networks and the third router would then be managed by both of you. Nobody has to let someone into his network or the password from his own router for port forwarding or whatever you want to do in the future. This is not optimally solved, but otherwise you can't avoid a second connection