RS300 and D7800 will not connect to ISP (to the internet)

Having spent  many hours trying varius configurations my problem was passed on to a more specialist team who have concluded that the RS300 is not compatible with the D7800 as a modem.

It turns out that the RS300 assumes the modem automatically sets the VLANID, as is the case in FTTP modems. They therefore did not include a setting for VLANID, (as in the D7800), and so it cannot tell the "dumb" D7800 which VLANID to use and how to connect to my ISP.

I would have thought it would have been easy for Netgear to play safe by providing the option to select the VLANID manually or choose to leave as automatic, (as in many other settings).

It seems that the Netgear developers believed the hype that the UK would have switched off all the copper, (analogue), as used in FTTC by 2025, (as it stands it could be, at least, 2027).
They should have known better! When have any big projects, such as this, ever been finished at the projected date?

Incidentally I have not moved to FTTP because my current FTTC is plenty fast enough, (speed isn't everything) and so see no need to pay the extra.
Speed does not come for nothing, there is not only the cost of the service, but the additional cost of running the terminal, (modem) it may only be a small amount but over 365 days it adds up to kWh. Don't forget that when the terminal or modem is on your premesis you pay to run it, not the network company as is the case for FTTC!

As it is, I have an RS300 which I cannot use as a router. It has, however, been pointed out that I can switch its Mode over to use as a Wi-Fi AP, so I can, at least, use the better Wi-Fi.