@Iain Jackson /@Tommi - We just went through a similar situation here and found that the old tenant was set up using a Service Principal.
Check Azure Active Directory > App Registrations > All applications to see if there are any Service Principals that were used to create your Classic deployment.
If there is, create a new secret and connect in this manner:
$aadtenant = "Directory (tenant) ID"
$appid = "Application (client) ID"
$secret = "Client Secret (value)"
$creds = New-Object System.Management.Automation.PSCredential($appid, (ConvertTo-SecureString $secret -AsPlainText -Force))
Add-RdsAccount -DeploymentUrl "https://rdbroker.wvd.microsoft.com" -Credential $creds -ServicePrincipal -AadTenantId $aadtenant