What can I do with a Motorola 6121?

[jcglz58]

With comcrap you only need to hook up the modem, wait until get ip and then try to open internet, intermediately is going to redirect you to their website where you only need to confirm your current account number and information about you. Few minutes later its all done and ready to surf...

Now, can you check in the configuration page of this modem in the log event section what exactly are you getting when your modem is hooked up to the coax?

Me: Hi, I would to install my own cable modem and cancel the modem rent.
ISP: Ok, you can use your own cable modem but you have to install it yourself.
Me: How can I do that?
ISP: Just connect the power and the coaxial cable and that's it.
Me: But I already did that and the modem can't get an IP address, I think you have to allow my HMAC or something.
ISP: I can't help you with that, we used to do that about a year ago and charge for it once but now this service is suspended.
Me: But how can I use my modem then?
ISP: Sorry, we can't help, you have to install it yourself.

If I had to presume anything, they are inferring the new unregistered modem should bring up a captive portal page with a login to automatically register it.

No idea why yours doesn't work as such.

You shouldn't need to hack your modem to achieve such, more of a customer service issue.