Multiplayer not possible?

I moved to a new apartment where I don't have access to the router. I can't connect to Steam over the WLAN, let alone play with others. TeamSpeak and WhatsApp calls also don't work. Skype, on the other hand, works perfectly. I have no problems with streaming content such as Netflix, Youtube, etc. And normal browsing.
It is not because of the PC, because I can use the Internet to its full extent if I connect my computer to the Internet via the mobile data of my cell phone and the problems also occur on my laptop and cell phone.

What can be the reason, or can I do something to fix these problems?

Ask the admin whether he releases it or concludes his own internet contract. 😅

Sounds like it's all being blocked. Either from the router or from the provider.