What do you need to configure so that your connection switches over to the stronger AP?

There are two relevant settings you can configure them on the NXC.

1. Enable RSSI threshold and configure the values.
Go to CONFIGURATION > Object > AP Profile > Radio > 
To select 2.4 or 5GHz radio profiles and click "Show Advanced Settings" and then enable Signal Threshold and fill in the signal values on Station Signal Threshold and Disassociate Station Threshold.

Station Signal Threshold is to set a minimum client signal strength. A wireless client is allowed to connect to the AP only when its signal strength is stronger than the specified threshold.

Disassociate Station Threshold is to set a minimum kick-off signal strength. When a wireless client’s signal strength is lower than the specified threshold, the NXC disconnects the wireless client from the AP.

For example,
Station Signal Threshold: -70dBm
Disassociate Station Threshold: -80dBm 



2. Adjust the power settings of 2.4GHz or 5GHz , 
Go to CONFIGURATION > AP Management > AP Group 

So you can decide to change the power settings of 2.4G or 5GHz are not maximum to avoid the client connect the signal is still good with far AP.

Thanks!