If I understand correctly, you know how to do port forwarding and that using different ports, the forwarding works. Some suggestions:
From a LAN side device can you get to the desired web server? That is, first insure the LAN component is working.
If possible, change the LAN side web server not to use port 80.
Set up logging of all your port forwarding rules. This will show whether the router is actually receiving the incoming requests from the Internet or if the ISP is messing with things. First use the existing port forwarding to see what the logged data looks like, then test the bad one. If nothing else, this can serve as an audit log of the forwarded port.
If all else fails, have the router trace every bit and byte using the support.cgi web page. This pcap will really show if the router is in fact receiving the incoming requests on the port. In this case, you will need to look at both the WAN and the LAN pcaps. At one point I was getting strange results with port 53 and this pcap trace showed the router was never getting the request, an ISP was intercepting it