This is a small Minecraft server for a few friends that was set up with a tunnel style connect method instead of normal router port forwarding. It was made with a launcher that runs a vanilla plus kind of setup and it worked fine at first.
Then it suddenly stopped letting anyone join from outside and only the owner could get in using localhost. The problem ended up being a free plan data limit getting hit and it looked like the server just broke.
The fix here was switching to a different tunnel service and it started running smoothly again. This setup is also meant for people who cannot easily change router settings because of a double NAT situation so it is a good option if you are stuck like that.
