Every time someone starts a server up, it seems to require a new ip and sometimes a new 5 digit number with it.

Is there a easier way to just join rather than having to look up the new one every time and get it from the maker.

Wireless modem that is not connected to internet ( offline ) lan.

Yes. Configure that router to static IP addresses. Sounds like it is configured to give the same system a different IP each time it connects. Change that, however that has to be configured with this specific router, and the same PC will always have the same IP.

Every device on the network has its own IP. If someone else attempts to host, the IP is different, because it is someone else. This solution only works for when the same device hosts again, but turns out to have a different IP than it used to have last time.