Port forwarding is by far the easiest solution.
Reverse SSH Tunnel
Another way would be to involve a third-party publicly accessible server as a relay. You'd have to initiate a connection to this third-party server from the web-server (since your router blocks inbound connections) and then use that connection to relay HTTP traffic via the third-party server..
This can almost certainly be done using SSH tunnelling
I frequently need to get access to a machine behind a firewall to do
some web development, but I don’t have a VPN available. Not a problem
– just use a reverse SSH tunnel. By the end of this tutorial, we’ll be
able to SSH to the remote server, as well as view any web pages that
server has access to.