What are the possible ways of accessing a web server without setting up port forwarding? (or basically, no access to the router - so no DMZ). Reverse Proxy?
You haven't really provided many details, but I'll give it a shot.
A reverse proxy would have to run on your gateway to meet your requirements...otherwise, you would still need to set up port forwarding so that the proxy could reach your web server.
A VPN solution might be appropriate, depending on what you're trying to do. For example, I have various services running at my office that (a) I want access to from home and (b) I do not want to expose to the public. So I connect from home to a VPN, effectively putting me "inside" the office network so I can access the services directly.
If you give us more details about exactly what you're trying to do we can probably provide a more appropriate answer.