Typical setup - should work with the nano.

First re-enable your router firewall.

I assume your nano has a static address on your network, and is on port 80

In the router, TCP forward external port 80 to internal port 80 on nano internal static IP address.

If the nano when setup asks for dns server then put in the IP address of your router.

You will, most likely, NOT be able to access your web pages from any PC on your internal network by using your external IP address, get somebody else to try it.

This is not a router problem just how it works.