I somehow fixed it, and I have no idea why it works. LiteNetLib works correctly and does not throw errors in my friend's console anymore. Also in the server console, it appears to be used for the connections without issues. When spawning 100 zombies, although my friend's pc lags badly it is because of his pc, the connection does not skip ticks anymore, and there are no sync issues.
What did I do? Nothing except forwarding two ports for my server's IP: 443 and 80. These have nothing to do with online gaming from what I know, but if it works...