Can't connect to Minecraft Realm over wi-fi in Germany

I can't join my realm with my smartphone via wi-fi.

Joining the realm with 3g / lte is still possible.

I even forwarded all ports via my Router (FritzBOX) so that it was fully exposed to the internet with all ports open, but i still can't connect to my realm.

Switching to 3g / lte let me immediately join the realms server.