Port forwarding is broken

I need to port forward to a static IP address in my network. I tried to do that, but the only choices for devices are DHCP devices that the router has given addresses to. Fair enough, I can use IP reservation - so I entered a few test addresses which didn't work. Now when i try to delete them, I get an error. Tech support was unable to come up with a fix. They escalated it and after hours on hold I gave up.

Has anyone else successfully deleted a port forward, or successfully forwarded to a static IP address? My only choice may be to return this modem to Xfinity and buy my own.