I've run into same issue, that "Win Device Admin Agent" services was not registered under services.msc on my MTR. With this script I was able to install it and my device appeared in TAC a minute after installation.
# run this script from elevated cmd if you do not see the "Win Device Admin Agent" service configured on your MTR under services.msc
# powershell.exe -executionpolicy unrestricted -file "c:\<folder>\install_agent_service.ps1"
# script variables
$basePath = Join-Path ${env:SystemDrive} "Program Files"
$agentPath = Join-Path $basePath "AdminAgent"
$serviceFileName = "WinAgentSvc.exe"
$serviceName = "WinDeviceAdminAgent"
$serviceDisplayName = "Win Device Admin Agent"
$serviceDescription = "Teams Admin Agent for windows."
$agentServicePath = Join-Path $agentPath $serviceFileName
New-Service -name $serviceName -Description $serviceDescription -DisplayName $serviceDisplayName -binaryPathName $agentServicePath
&sc.exe failure $serviceName reset= 300 actions= restart/60000/restart/120000/restart/240000
Start-Service -name $serviceName