Hello KrysK
Try disconnecting from the modem WIFI network and reconnect and check for the connected device on the modem interface, If still you see the devices are not showing on the interface try restarting modem and give forget network and again reconnect after the modem reboots. You cannot add device IP address manually if the devices are not connected to modem WIFI network.
I rebooted modem, and started to report false IP addresses for my print server, if I set it to .122, it would go up by 1 to .123, and continued walking to .128. In week’s time, suddenly returned to original .122 and appears to stay there now. I had a device removed from the system that had address ending with .49, and there is now way to delete it to recover address. Clicking on X in Online Devices removes the device to Parental Control (?) / Managed Devices. Clicking X to remove device restores the device back to Connected Devices. Full circle. Just wonder about Arris’ QC, does anyone test software for loops and idiotic behavior?