@AlyJay
I fitted a vision plus to my first van, never again absolutely appalling product, exactly the same problem as you described it would give the impression there was signal but often absolutely nothing unless in a really strong signal area with good line of sight
I also messed about horizontal vertical pointing it 180 out etc.
On both my current vans I now use the truly incredible Necvox magnetic antenna, if there’s signal out there these will pick something up where the vision plus would not.
I also have a fire stick however for 100% reliable tv I carry a portable satellite dish in the vans as backup, for when there’s no mobile or terrestrial tv signal often there’s not even radio 4 in large parts of the UK!