Manually assigning a Remote Desktop (RDP) certificate
If a Remote Desktop certificate was requested manually , it must then be assigned to the Remote Desktop session host.
Procedure
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First, the SHA1 fingerprint of the (most recent) Remote Desktop certificate must be obtained. This can be done with the following Windows PowerShell command:
$RdcCertHash = (Get-ChildItem -path Cert:/LocalMachine/My | Where-Object { $_.Extensions.EnhancedKeyUsages.Value -eq "1.3.6.1.4.1.311.54.1.2" } | Sort-Object -Descending NotAfter | Select-Object -First 1).Thumbprint 
Then, the certificate can be assigned to the Remote Desktop session host using the following Windows PowerShell command via WMI.
$TerminalServicesConfig = Get-WmiObject -class "Win32_TSGeneralSetting" -Namespace root\cimv2\terminalservices -Filter "TerminalName='RDP-tcp'"
Set-WMIInstance -path $TerminalServicesConfig.__path -argument @{SSLCertificateSHA1Hash="$RdcCertHash"}