You need to keep phoning the FTTP team 0800 587 4787
It sound like the serial number of the ONT wasn't submitted to BTWholesale when it was installed. Might be worth giving it to the FTTP team and asking them to submit it.
I am in the exact same position and share your thoughts regarding BT. I have been chasing for three weeks now - my modem was fitted at the start of December and still no real response or reason given for the delay.
Absolutely shocking service.