I would agree with you that the port forwarding is the issue. Because it depends on if you are trying to access the install from outside the system or from inside the firewall.
You stated that your ISP doesn’t allow certain ports. And if that is try from the outside of the system there is no way around that in my opinion.
You might want to use a port detective software to see what ports you do have open.
Question:
When on the same side of the firewall as the Ubuntu server can you access the software from
localhost:8080/openemr?