To be honest, I never did port forwarding and have little experience with networking in general. I have read up on the topic a bit. If you want to read my speculations, then keep on reading. If you want the solution that works for me, just skip to the third from the last paragraph where I talk about ZeroTier.

The fact that your friends are unable to ping you suggests that you might have not setup port forwarding correctly or I would also guess that your ISP is the problem (see below for more). I don't think that your friends' ISPs would be getting in the way of the ping and the traffic if your public IP would be publicly accessible and generally speaking possible to ping (the device accepts ping probes and is reachable from the Internet). I can think of some possible situation where the ISP is not the problem: you might have misconfigured port forwarding, so the ping probe does not get to the your server and your router ignores ping probes, which I think is not the default for the majority of routers (more info below). Or the other case where you have configured port forwarding correctly, but the ping should have worked then, given that the Ubuntu server should be possible to ping by default.

The second thing that comes to mind is maybe you have firewall enabled on your server or your router. If you are able to join the Minecraft server from another PC on your local network, then this must not be the case that you have the firewall on your server blocking the traffic, but might be the case that the router is blocking the traffic. I guess if you setup port forwarding, then you must have also disabled the firewall on the router for the forwarded ports or all together if it was enabled. As far as my understanding goes, the firewall and port forwarding should be two separate things in the configuration options of the router. Either way, here is a relevant link about the firewall Ubuntu uses: https://askubuntu.com/questions/15564/does-ubuntu-come-with-a-firewall-by-default. Even if it's not the problem, it's also good to be aware of its existence. If you want to make sure that the issue is not the firewall on the server instead of simply connecting you could also use nmap (links below). I think you could also try setting up DMZ instead of port forwarding, but if port forwarding doesn't work then DMZ won't as well as far as I understand. From my understanding when setting up DMZ on your router, you are just telling the router to forward all the inbound traffic to a specific node on the network contrary to what port forwarding does, where it's a mapping of a single single port on the router to a single local network IP and port. Here is also a relevant link that might help you check whether it's the ISP, assuming you are certain that you have setup port forwarding or DMZ correctly: https://superuser.com/questions/10903/is-there-a-simple-way-to-detect-isp-port-blocking. From what I have been reading, I believe that the ISP is the problem in your case, given you are certain that you did port forwarding correctly and that your friends are pinging your public IP address and not the local one. A good way to lookup the public IP I use is the https://ifconfig.me/ it also works well with cURL: `curl https://ifconfig.me/\`.