TX power is always a tricky thing as you want to allow for a slight overlap otherwise you have dead spots but  not so much that devices connect to the wrong AP as in your case. 
 
If this is a simple work from home I would unplug the loft AP and the adjust the TX on the ground unit until the loft devices can only just see it, then do the opposite. 
 
If it's a larger office then you will need a site survey. 
					
				
			
			
				
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