Buying a car with pre Brexit NIE

Hi, 

if anyone can help us clarify this question, we'd be very grateful.  

We got our NIE number in September 2018 when we bought our flat here in Ciutadella de Menorca, whilst we've paid our council tax (impuestos municipales), electricity, water etc., regularly,  we haven't to date managed to get our empadronamiento, we tried last year but found it dificult to navigate the bureaucracy here, and gave up

Now we are buying a car, but when the Owner went to register us as new car owner at Hacienda in Mahón, the person there said that we were not registered with them, and that we need to go there. 

 Before we go with the owner we wanted to check if there is anything we should know about, like if we should have renewed our NIE number, due to all the changes since Brexit? 

thanks

Anita