Yes port forwarding works well with the mobile app.
When using port 80 you don’t need to define it. See if that helps (just the IP).
Are you sure your port forwarding port 80 to the correct device?
I tried just the IP same problem. I’ve got a Linksys router. My OSPi has a static IP of x.x.x.131. In the router I went to ‘Single Port Forwarding’ There is an External Port and Internal Port Column. I set them both to 80. Protocol is listed as ‘Both’, then my .131 IP address with the enable button checked.
I disbale wifi on my phone, go through the steps but I still arrive at ‘Check IP/Port and try again’
Are there any other settings needed in the router?