Duo + Remote Desktop Gateway + On-Site Computers

I’m supporting a client that is using Duo with Remote Desktop Gateway. The Policies within RD Gateway are not available, instead NPS is used (which I read on the forums). However, they are now unable to RDP to on-premises computers via RDP. RDP Web works just fine with session hosts. Authentication to RDG works, but it gives the error “The remote resource can’t be reached.”

Is there any way to allow remote resources through NPS with Duo Authentication installed on the Gateway server?