Hi,
Thanks for posting in Q&A platform.
If you want to forward connection requests to a remote NPS or other RADIUS server, create a Remote RADIUS Server Group and then configure a connection request policy that forwards requests to that Remote RADIUS Server Group. With this configuration, NPS can forward authentication requests to any RADIUS server, and users with accounts in untrusted domains can be authenticated.
Please kindly check if the following articles were helpful:
https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/cc755217(v=ws.10)
https://learn.microsoft.com/en-us/windows-server/networking/technologies/nps/nps-plan-proxy
https://ittrainingday.wordpress.com/2014/01/03/how-to-configure-radius-proxy-servers/
Please Note: Since the websites are not hosted by Microsoft, the links may change without notice. Microsoft does not guarantee the accuracy of this information.
Please understand, due to the environmental limitation, we have no such third party Radius proxy to test in our lab. It is hard for us to reproduce and analyze this issue from Q&A platform support level.
Hope my answer will help you!
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Best Regards,
Sunny