Cant install custom firmware on ISP routers

Hi folks

I looked around for custom firmware for several models of modem I picked up for nothing, and I couldnt find anything because of ISP custom firmware on some belkins and no name ISP models. I also found a billion, a linksys and so on but the models arent related to custom releases but have similar models there

Is it possible to install a similar model number on the modem if one doesn’t mind bricking the modem? Are there other options that can be explored if the specific custom firmware doesnt exist for your model? Could it be possible to install say puppy linux and run your own customized linux OS on a router?

cheers