85 votesDimarc67 commented
Encountered same issue. Windows Defender Firewall on WinSrv2019 blocked RADIUS auth and acct ports UDP/1812-1813 despite the Inbound rules being enabled (default).
Resolved this by changing the Edge Traversal setting for existing Network Policy Server rule from "Block edge traversal" (default) to "Allow edge traversal". (In my case, the traffic is coming from a different subnet, so "edge traversal" could be in play.)
RADIUS authentication now successful.