Radius Load Balancing with UTM9 for Enterprise Authentication (Wireless)

Hey Forum,

I'm fighting with the configuration of our UTM9 when it comes to Radius Load Balancing.
The goal is not to design a fail-over (with availability groups) (which is easy by the way) :P
We'd like to load balance the radius traffic with the UTM Server Load Balancing System.

So my idea was this:

But unfortunally I get an

when I try to test the Authentication Service.
When I select one of the radius servers directly (without load balancing) everything works fine.

Can you help me to set up the load balancing part?
Via terminal i get an:

$ nc -u -v ip-of-the-virtual-server 1812
Connection to ip-of-the-virtual-server 1812 port [udp/radius] succeeded!

Has it something to do with the shared secret?

Thank you!