Thank you both for the help!
Alas my ISP does indeed have CGNAT so I'll have to emulate LAN with one of the recommended Apps.
Just as well because my router didn't allow for a range of values for the internal port. No symbols other than numbers were allowed, not even spaces. Leaving it empty would just add 80 automatically like the external port.
I thought maybe the firewall on my anti-virus was to blame but even after turning off the anti-virus, firewall and all, it still would not take.

But thanks once again for your help.