Accessing External IP from LAN through an access point

Solution 1:

I'm guessing this is a problem with your DNS records or settings somewhere, which is quite common when hosting sites in multiple networks (i.e. internally and externally).

Perform nslookups on your site while connected to each network (modem, router, externally). Externally you should see your public IP, internally you should see the private IP of the server (192.168.x.x). If you're seeing a public IP while connected internally, that network isn't configured to use your internal DNS.

If, however, the situation is that you can access your site via the public IP internally while connected to the modem, but not while you're connected to the router, my guess is that the router is configured as a wireless AP, rather than a bridge. I.E. it's NATting packets sent from ethernet across the wireless bridge, rather than just switching them. In this case you'll have to adjust the settings on your router. Make a backup before changing anything, and download the manual before you lose internet.