i ran into this DHCP scope issue on one of these units last night. if you change the IP address of the modem, the existing DHCP scope does not automatically change to match. you can add a second scope but there is no way in the GUI to delete the first 192.168.1.x scope.
turning DHCP on or off makes no difference and didn't make the telecom-supplied scope disappear.
this was a pain for the customer as everything else on their network was set to a different IP range (network laser, NAS, playon HD, etc). so thanks for posting the link regarding making the change using telnet.
best wishes for the holidays to all.