If your only issue is with that specific device, you should should start by surveying that area to see exactly what coverage you are getting.
You should also look at the RF utilization using the controller's dashboard to see if high utilization is keeping that display sign from connecting effectively.
The problem with having an access point at the power of 24 is that it is incredibly high. The AP power, dBm is logarithmic, so every 3 clicks you go up, effectively doubles the power. Having power that is too high is the biggest reason for (1) roaming and (2) having clients that bounce between access points. In addition if the power is too high on the access points, clients end up talking to access points that probably cannot hear them and that will decrease performance considerably.
Take some time to measure what is in that area to get a full picture of your issue.
- 0dBm = 1mW
- 10dBm = 10mW
- 12dBm = 15mW
- 18dBm = 63mW
- 20dBm = 100mW
- 21dBm = 125mW
- 22dBm = 158mW
- 23dBm = 199mW
- 24dBm = 251mW
As a reference iphone5 = 13dbm max output