What browser are you using?
Try Opera or FF? If IE 8 or 9, set compatibility mode and test again.
Turn off all anti virus and firewall programs on PC while testing. 3rd party firewalls are not generally needed when using routers as they are effective on blocking malicious inbound traffic.
Turn OFF ALL devices. Start with 1 wired PC to manage the router. See if it is displayed under NUMBER OF DYNAMIC DHCP CLIENTS. I believe you need to use the drop down menu (<<) under Add Reservation section above and this should list the available device(S) to add. Select (<<) to fill in the boxes. Change the NAME and or IP address here. Select ADD then Save Settings. I think this router supports Reboot Later. Try reboot later then turn on another device. You might have to refresh the web page or navigate to another page then come back. If you don't see the newly added device, try to reboot the router.
Let us know if this process works.
Maybe someone can review your router settings with you using Link>
teamviewer if your interested. Its safe and secure.