How do I delete Offline Devices that show in Offline Devices but they are online and pingable?

I have an ADT home security system with cameras.  Before this new garbage of doing things through the XFinity website or app, I managed everything well right from the router.  Now for whatever reason, my ADT cameras are showing in the Offline Devices section, but the device is online, pingable and I can access the cameras from my computer.  I need to configure port forwarding for them, but since the device is not found as being online for whatever reason, I can't enable port forwarding directly to the IP address without selecting something from the list and that's ridiculous.  I've confirmed the IP and MAC address are both correct, so I do not know why the router thinks the device is offline.  So, I wanted to delete it and recreate it with the static IP, but as usual, that is not an option.   Comcast, why have you made this so difficult?  I've seen this issue with other people and they never get a straight answer.  Just tell me two things - 1.) How does a device that is online and pingable end up on the Offline Devices list?   and 2.) How do I remove devices from the Online or Offline Devices sections?   Thank you.