DOCSIS in practice does not guarantee interoperability. Each ISP has its own supported modems list. Deviating from their blessed hardware means they will not send the special ISP specific firmware that’s bug compatible with their setup. Or they might refuse to activate it. Of course any issues are immediately blamed on customer owned CPE.

Because of this the real point of demarcation is the approved modem/gateway’s Ethernet port not the coaxial junction box.