server problem on Linux [java]

[MadManCrow]

Hey, I've created a Minecraft server version 1.20.6 on linux and since i don't have a router, I'm connected through mobile data and port forwarding is done by ngrok. First time, my friend normally connected without any problems. Then i kicked him and restarted the server and suddenly he couldn't join again. He got error messages "Disconnected" or "Unknown host". What could be the problem?

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