That shows that the address you have pasted the information from the checker for , already has FTTP installed , so either when presented with a list if addresses, you picked the wrong one , the main house and not the basement flat , or the main house residents when ordering their FTTP service inexplicably used your address ( the basement flat ) instead of their own address, or there isn’t two addresses a main residence and the basement flat , but one single address , and you picked the only one available because there wasn’t two to choose from .
This could mean the address doesn’t have two official individual units within it , otherwise it would have two entry's, one for the main residence and one for your basement flat, or the data OR use is wrong or out of date.
Its possible , if this basement flat was only recently created/ separated from the main residence , and has been correctly registered as a separate dwelling with the appropriate authorities, so it has its own a separate entrance , has its own council tax account, and has its own utilities ( separate gas, water , electric etc ) then the Openreach records may be historical whereas the Royal Mail record is up to date , if this flat has existed for a long time , then it’s odd that it doesn’t have an entry on the Openreach database as that’s pretty much a ‘copy’ of the Royal Mail dataset , so it could be it’s an unofficial flat , it’s basically just a room within a building.
If you are convinced that the flat is a separate ‘proper’ address , and not simply a room or rooms to rent , then the Openreach records need to be corrected to match the Royal Mail , assuming the Royal Mail website shows two separate addresses , the main residence and a separate basement flat .
