Might be a couple things creating this "perfect storm." I haven't read its manual but I'll stab in the dark.
1) You can log onto the router wirelessly but not administer except, it appears from your image, to add MAC addresses. No other admin options.
2) You can log onto the router wired and still not administer.
As far as wireless administration, there may be an option to enable wireless or remote administration. That's a security feature on some routers. A hacker may log onto your router but can't add, change or delete any settings via the wireless hack. If you want to administer wireless, look for this feature.
As far as the wired administration, you may still have to enter the MAC address of your desktop. I have an older router and haven't frequented newer routers, so I've only seen MAC addresses used for MAC Address Filtering. MAC Address Filtering permits certain device by MAC address to log onto your network. A MAC Address Filtered router would deny any connection of a device not listed in your Filter. Add your desktop MAC.