If your address ‘21b’ doesn’t appear, what address have you posted the result for ?, presumably number 21 or 21a .
I think your problem could be that if your property is new build and slotted in between existing property’s , when the area was surveyed for FTTP your property probably didn’t exist, subsequently, although it’s now built and registered with the PO/RM and liable for council tax etc, presumably OR records still don’t show it.
Its quite possible that an order can be taken for copper service on addresses that are less than a perfect match on OR’s address database, but perhaps not for FTTP.
Quite often on this forum the Mods are asked to arrange an ORDI ( Openreach database integrity) to match OR’s records to the PO/RM database entry, as address mis- matches can cause issues, but you have to be a BT customer before the BT mods can do this ( often it’s a Sky or TT customer for example asking )
As already suggested, try contacting the FTTP support team , they may be able to override the addressing issue and enter an order with a less than perfect address match, or it may require copper service to be provided first, forcing the address to be recognised, then an upgrade to FTTP to be issued.
