This is my opinion. - I think I understand how it is intended to work and how it is working although I am open to being proved wrong. I am sometimes but not very often

I have 3 AP900's - If I have the same SSID on each but not using Radius server it works reasonably well. But as you move out of range of one to another as I understand it goes through the process of authenticating to the new AP so there is a small period of time when signal is lost. As I understand with Radius server and roaming enabled (so you have to be using 802.1x in the security mode) it should pre authenticate with the other AP's and thus it should only have a very small time period of loss of connection. This would be the ideal and if you are using VOIP it should not drop the call as you move. However my experience of this is it just does not work reliably to the extent that the device often ends up having no wifi at all. When it works it is good but it fails so often to be unusable in this mode.

Jon

A little late to the thread but I thought I'd add my experience. The AP900's Radius server is flaky IMO. Every hour (Key renewal), wifi devices would drop their connection and refuse to connect. I gave up trying to troubleshoot, set up an external Raspberry PI as a radius server and it's been fine.

Thanks ijwoo1 interesting - I tried using a QNAP NAS Radius server in place of the AP900's Radius and it made no difference are you sure that is the only thing you changed?
Jon