@NgA -- it sounds like you're changing the external port you're forwarding from as opposed to the interval port you're forwarding to. It doesn't matter what external port you're forwarding from and point your browser at, if you're forwarding to 8080 internally, you're going to see the streaming service. However you setup your port forward to 8080, just change the internal port to 80 instead. It doesn't matter what the external port is so long as you put the same one into your browser/app on the external side, you'll see the OctoPrint web UI.
For example, if you already have a web server running on your gateway or you're already forwarding port 80 from your gateway to a web server internally for other purposes, you can still forward 8080 from your gateway to 80 on your OctoPi and then pointing a browser at your public IP :8080 should still give you the OctoPrint web UI.
Disclaimer: @WarHawk is right -- you're better off setting up a VPN and having some security in place. Putting OctoPrint (or anything really) directly accessible to the Internet is a bad idea unless you really really know what you're doing.
Even Bigger Disclaimer: Be wary of printing when you or someone you can contact easily is not physically near the printer. 3D Printers do occasionally catch fire in the real world. Someone needs to be there to deal with that in the unlikely event it happens. Ask me how I know.