Unfortunately modems have the issue that they are made to be provisioned, configured, and owned by the ISP. At least that’s how most DOCSIS setups I’ve seen work.

The only hope I’m seeing (I’m also wanting to do this) is running my own CMTS. I found a service (https://github.com/cablelabs/os-provisioning is the only opensource tool I’ve seen before) but I am still trying to figure out a reasonable cost way to connect that server to the actual network (so far bladerf sdr is ny best hair brained idea, which is really only cost effective because I want to play with 5g too). The last bit of luck I have is that my ISP isn’t actually doing firmware updates on my modems, so if I connected them to my Internet lab network, provisioned my custom OS and firmware, and then connected it back to the ISP I can get their DOCSIS config. In theory at least 😅

Netgear has modems that have their OS and firmware in nice packages with build instructions at least. ARIS’s older modem code is out there too but a pain compared to get.

You technically could use that same sdr as the modem on your ISPs network BUT because it’s actually RF the FCC regulates it (for good reason I guess lol).

If security is the actual I’d even argue you SHOULD over ride them. It’s like the default password on your home router