Friends can only connect while I'm on my main System

OS Name/Version: Ubuntu 26.04 LTS

Product Name/Version: AMP ‘Proteus’ v2.8.0.0 (Mainline)

Problem Description:

I created an Minecraft Java Server with Forge and and the Modpack MC Eternal2 on a seperate Mini PC with Ubuntu Server.

After that I used an Tunnel priveder so that my friends can Connecticut to the server. At first everything worked fine. They could connect to it. But 1 hour after I disconnected and shut my Main PC down (not the mini PC) they bot cicked Form the Minecraft Server and couldn’t Connecticut with it.

They are getting the message: connection timed out:getsockopt

At first I opend my IP Andreas directly without a proveder with exactly the same mistake occour. After that I tried the Tunnel provider playit.gg. The Adresse Form the provider is funktional and doesn’t change.

So my question is, what I do habe to change in the settings, that I don’t need my Main system where I Connecticut with the mini PC and usw AMP (AMP is oft corse localy installed in the mini PC)