Various factors influence the client's decision of which AP it should connect to.
However, the client's decision can be influenced by using the correct configuration settings. The decision for which AP to connect to is completely made by the client and there is no way to force a client to connect to a particular AP. The initial association process is explained in the 802.11 Association Process article. The access points act as a bridge between devices and the local network infrastructure. Meraki Authentication uses a Meraki hosted RADIUS server, and testing with this may be helpful for identifying local or client-side RADIUS issues.
Meraki Authentication can be used as an alternative to RADIUS Authentication for testing as the basic functionalities are similar. This also prevents disturbing the entire network when only one AP is in question. These settings will remove all the third parties involved and make it easier to diagnose the issue between the client and the access point. If you want to contain your test, go to Wireless > SSID Availability and t ag the SSID with the AP's tag so that only the AP in question broadcasts it.