Is the PON light on the ONT lit ( steady green ) , if it is , then really all the ISP needs is the serial number of the ONT to identify that particular device on the PON , if you want to use BT , you could call the FTTP team and they may be able to raise an order manually, ( the regular customer service reps cannot help, but the FTTP people usually can )
OR before the building is constructed, are given the plot numbers by the developer ( presuming the developers were in touch with OR early in the development ) when OR fitted the ONT they primarily look for the plot number, there can be issues with house numbers and plot numbers being mismatched, but the post code is relatively insignificant at the stage the ONT is fitted .
Obviously, a mismatched post code could impact getting an order raised with an ISP if the postcode is incorrect on OR’s records , have you checked the information the Post Office have is correct, as that is the basis for OR’s records.
