Dave Bauman, you do ideally need an internal AD CA that issues certificates to your NPS servers and probably your workstations and DCs too. and the CA is in Trusted Root Certificate Authority on the connecting workstations ( which an AD CA Cert is automatically added to by AD to all domain joined workstations.)
to issue to workstations...
GPO Computer Config > Policies >Windows Settings> Security Settings > Public Key Policies/Automatic Certificate
Request Settings > Automatic Certificate Request > Computer...