Can you also play Minecraft Multiplayer on the mobile phone with the flat rate because in my WLAN somehow it doesn't work with multiplayer.
Why should not that work?
Because then there's a message "you have to be connected to a Wi-Fi network to play with your friends" and now it doesn't come with "vlt you have no friends at all"
So if it is not a local lan server, it should actually work.
Maybe you have activated that you can only play over Wi-Fi (so that no mobile data is used)
Of course, local network games only work over Wi-Fi