Even if you buy your own modem they can push firmware to it (and do). The config file your modem downloads includes a cert that allows the isp to do this. You can flash special firmware (used to be called force ware) to prohibit this.
However, I would assume no unencrypted traffic is safe anyway, and the modem would indeed not have access to your internal network.
The firmware on whatever is doing docsis is going to be updatable by the ISP generally.
Two different mechanisms. The tr069 management and snmp triggered firmware upgrade