Recently trying to splin an enviroment up in DR and upon login to the RDS Gateway , I was getting the below

Logging into the RDGateway showing the following event from the RDS Gateway

“The user “DOMAIN\Username“, on client computer “IP”, did not meet connection authorization policy requirements and was therefore not authorized to access the RD Gateway server. The authentication method used was: “NTLM” and connection protocol used: “HTTP”. The following error occurred: “23003”.

I had a look on the NPS Role ( Network Policy and Access Services” and saw that the user was trying to be authenticated locally instead of the remote Radius Server that was set in The RDGateway

“Remote RADIUS (Remote Authentication Dial-In User Service) server group does not exist”

I rebooted the Gateway and this fixed the issue , I think it had come up before the domain or radius serverv.