You need to disable remote management or alternatively change the port on which remote management is published. Your Webserver presumably uses port 80 (which is also the default port for the router) so if you change the remote access port to (for example) 8080 you can still access the router by using >
http://my-ip:8080 but >
http://my-ip/site will work as before.
The remote access settings are accessed on the Advanced / Remote Management tab of the router's web interface. If you don't want/need remote access to the router's web interface then you can turn it off by unchecking the "turn Remote Mangement on" checkbox.
You probably know this already but rather than having to know your router's external non-fixed IP address in order to access your webserver, you can use a free dyamic DNS service (dyndns.org) to give your webserver a proper address in the format of mysite.dyndns.org. If you have your own personal domain you can then redirect calls on port 80 to that address so that
www.myname.co.uk is redirected to mysite.dyndns.org and then onto >
http://mysite/site
Just use the Dynamic DNS page on the advanced tab of the routers interface having signed up for the free service.
Hope this is useful