100% a upnp issue. either your router’s upnp was disabled or the people who cant join dont have upnp enabled on their routers anymore.
if you port forward anyone can join. if you dont, only if both you and the joiner have upnp enabled will the server be joinable
ip change is irrelevant as the server sends its current ip to the master server for a client to use to attempt to join. however it may be tier to the router settings getting changed/router getting reset and thus upnp being disabled