XB6 - how to set static DHCP reservation for LAN devices ?

I have the Comcast XB6 xFi gateway configured in router mode. I have the DHCP range set to 10.0.0.2 through 10.0.0.253 . I'm trying to assign (private) static IPs to my (many!) LAN devices.

1) First, I go to "Connected Devices -> Devices"

2) Then I choose a device and click "Edit".

3) Then, I change the configuration from "DHCP" to "Reserved IP", choosing a new, previously unused IP address from the DHCP range, 10.0.0.50

4) I click "Save"

5) I get "Operation in progress" then after a few seconds : "Failure! Please check your inputs."

I tried to modify the DHCP range from 10.0.0.2 to 10.0.0.30 .

I repeated steps 1-4 above. Then I get "Reserve IP address is not in valid range: 10.0.0.2 - 10.0.0.30"

I tried another IP address from the range, 10.0.0.4. And now I get again "Failure! Please check your inputs".

Does this feature work at all ? And if so how ?