Just to make sure, I recommend calling their ISP provider to know exactly what you are giving up in terms of support, how much TS they can do to a cx owned modem, in which situations they will need to pay a technician appointment for an issue by having their own modem and the cost of the technician appointment

ISP agents know very well the leased modems but they may not know all the tricks on a owned modem, so always have their owned modem manual available, the manufacturers contact number, and most likely agents will not provide any assistance related to login to the modem settings and handle their wifi network, ports, etc, which your dad may be able to as you mentioned he understands technology.

But yes my advice as tech support agent of another ISP is to call them an know the policy they have regarding owned modem, and before going to their owned modem make sure everything on their network is working on point while having the leased equipment