Neither of those work, basin wrench does not open wide enough nor does it grip a slim nut ( around5-6mm) and not enough room for it .
Box spanner can’t be used around flexi hoses which have to be fitted first .
Is there a similar tool to this with wider jaws?
Have a new basin tap and it has a single large nut underneath but standard wrench is 4-5 mm too narrow.
Never needed a dehumidifier, never had a mould problem.Opening windows is a poor way to ventilate in cold weather, reduces internal temperature resulting in more condensation especially in bathrooms.
The clamps are to hold it down not to pull it down so it would suggest that the hole isn’t quite large enough to accommodate the sink and is binding at the sides.
You will continue to have mould where clothes are dried indoors regardless of which room , the damp air will migrate . Adequate ventilation is required and it’s free unlike a dehumidifier .