DHCP Reservations: "Failure! Please check your inputs."

I am convinced that if the Arris modem/router were invented a few millenia ago, it would have been one of the plagues unleased on Egypt.

First problem.  The DHCP reservation system only allows DHCP reservations in the active DHCP address range.  However, if you go to set a reservation, it is likely that some other device already has the address that you want.  But even if it doesn't there is still a chance that the modem will reject your request for no known reason.  OK, fine.  I guess that I can live with this limitation, IF ONLY IT ACTUALLY WORKED!

I have some servers that I want to have a fixed IP address.  With my old Netgear modem, I set random DHCP requests to get address above 100, and leave below 100 for various servers.  I can't do that here.  OK, fine.  I will just work around.  But I go do "Edit Device" and change to "Reserved IP."  What do I get?  "Failure!  Please check your inputs."  The ONLY inputs are an IP address (which is within the address range), and a comment (which I don't use).  There is NO REASON that this should be rejected, but I cannot set a DHCP reservation to my Raspberry Pi.  What a useless piece of junk.