1. You don't have to be in DMZ, just make port 80 forwarding to apache server.
2. Router conf-->Miscellaneous Configuration:
"HTTP Server Access: This field allows the user to configure the Web pages can be accessed from.
All: When this field is checked, it allows both WAN and LAN access to the Web pages.
Restricted LAN: This field allows the Web pages access from LAN side.
Restricted WAN Specified IP & Subnet Mask: This field allows the Web access from WAN side with a specify IP and subnet mask.
HTTP Server Port: This field allows the user to specify the port of the Web access. . For example, when it is changed to 1001, the HTTP server address for the LAN side is [10.0.0.2:1001....]
This is from manual. So i suggest to restrict web management access to LAN only (it's good for security too). And hopefully this will fix problem.
If probs persists, then change router web management built-in http server port to 81 or something else (diff from default 80).