As I said docsis is often MAC address controlled, so if you can't get the admin password to unlock the devices (http://portforward.com has lots of isp configs/auth details) you could buy your own docsis modem/router that supports configurable MAC address and simply spoof the isp provided device.

For fibre based devices usually you just have a ONT with an Ethernet port, so no modem required just plug in your router.

If you can give more info on the specific last-mile tech you're dealing with, we can probably make some suggestions.