I have a Technicolor CGM4140COM. Great modem/router by the way, works great!
Intitially I set up a few devices with reserved, internal IP addresses, no problem. Recently (Feb 2019), I attempted to add a reserved IP address for a new device. It goes through the process and seems to be successful. However, when I go into the connected devices list, I see that the device does change to the new IP but there is no indication that it is reserved. It is still labled as DHCP, while my earlier reserved devices are labled "reserved IP". Am I missing something? Is there a limit on the nuber of IPs that can be reserved?
Did you restart both the gateway and the device after you reserved the IP address?