Depending on AP firmware version and the capabilities of the haldheld client, you may need to try out decreasing the minimum bit rate to something less than 12Mbps (try 11Mbps) and/or disable 802.11r on the Wireless > Access Control page for that SSID in case the client doesn't support 802.11r and try that as well.
Can I set 11Mb fixed on MR Access Point When I trace the ping I have this result: Start ping on Handheld --> AP (OK) --> Server (OK) --> AP Eth-Port (OK) --> AP Wireless Port (error) --> Handheld (error) I hope understand.
Well, this looks like more like a connectivity issue. Can you describe the settings you have on Wireless > Access control? If you have a password there is no need to post it here. What I would like to see what settings you use to make a recommendation based on that.
How far away are the end devices from the Wireless APs?