Realm

My friend and i tries to play realm together but he lagg out all the time and get the messege

"Connection lost" "internal Exeption: java.IOExeption:
an existing connection was forced to turn off the remote host computer "

What can we do to fix this? He has stable internet connection and everything else works. Like singleplayer, multiplayer and other games.

he have tried everything we found on google but nothing works.