Are you using EAP-TLS certs etc for authentication to NPS ?  We have seen similar issues at a few of our sites caused by some sites having lower MTU values resulting in packet fragmentation when using certs and VPN connections.  We tried lowering MTU packet size on NPS connection policies without any success.   Ended up using Meraki Radius Proxy for those sites with the issue - as it's sent directly out over the internet for this, then you don't get the VPN encryption overhead and the auth packets are not fragmented   Good luck ... View more