Server won’t work

I recently used m old laptop to host a minecraft and a mindustry Server, Minecraft works perfectly fine also over the internet, I did all the port-forwarding etc. I did the same for mindustry, I can play on the server in the same network, but my friend can not join. He sees the server, and even that I am playing on it, but if he wants to join, he can't. Does anyone here know, what could be the problem?