Port Forwarding
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.
(my emphasis)