What can be done to remotely monitor the Defender Security Server to ensure that it is up and and answering RADIUS authentication requests?
Monitoring the RADIUS port that the DSS binds to would only partially monitor the Defender Security Server.
By using a third party RPC monitor application, you could verify that the DSS is running and listening on the configured port. However, this type of RPC monitoring would not verify that the DSS is processing RADIUS request and authentications.
To verify this functionality, a RADIUS authentication would need to be performed.