From what I’ve gathered, this issue is prevalent in other games with very similar connection problems. This issue with the offline mode for multiplayer usually involves the port forwarding settings on your modem to be opened up for a specific port or ports involved with any multiplayer game that is based on or for Peer to Peer (P2P) system. Incase people aren’t aware, or don’t know, Peer to Peer is a person to person host, rather than Generation Zero being the soul host and we connect to them. Generation Zero or Avalanche doesn’t have an official hosted server we all connect too and host a game to us. In fact we’re the hosts of our own games, hence the name Peer to Peer connection (P2P). The server we connect to is an informational log server, it’s just to keep and maintain our gear and stats, and to implement changes via updates. I had to do something similar years ago with Warframe, as their system is the same P2P. They don’t have a host server as you’re the host, so people connect to your connection and play with you. Seem the standard ports for multiplayer P2P are UDP ports 4950 and 4955. Which could be why I’m able to have players join me, but I cannot join them because they’re on a different port.
Might shed some light on the subject. Might also help some people too. A little google searching doesn’t hurt anyone.