Here is some material I reference when working with RADIUS authentication.
I plan to post a basic guide here on implementation, as time allows.

Logging format : You’ll need this reference to translate logs. Windows NPS stores logiles in “c:\windows\system32\logfiles\”. They can be useful for troubleshooting.
Reason Codes : Translate the codes from the logs above.
RegEx Usage   : Get your Regex on in NPS policies.
Radius Packet Format :  Fundamentals.
General NPS : Microsofts general NPS admin documentation.

I have the log files shipping into Elasticsearch/Kibana, translated by Logstash along the way. I’ll post a guide for this eventually, along with the configs etc.