I would suggest looking into using a RADIUS service profile. I would think that using the Datagram LB option might cause different parts of the RADIUS process to go to different servers.
"The BIG-IP system includes a profile type that you can use to load balance Remote Authentication Dial-In User Service (RADIUS) traffic.
When you configure a RADIUS type of profile, the BIG-IP system can send client-initiated RADIUS messages to load balancing servers. The BIG-IP system can also ensure that those messages are persisted on the servers." Taken from Here.