So Microsoft changed things again. This time, it seems you can have a Teams meeting with external users but can’t chat with them, if their account is not company-managed. Here’s the work around for that, in Powershell. Please authenticate with your Tennant as usual and:

Install:
Get-Module -ListAvailable -Name MicrosoftTeams
Install-Module MicrosoftTeams
Connect-MicrosoftTeams

Set:
Set-CSTenantFederationConfiguration -AllowTeamsConsumer:$true

Check:
Get-CSTenantFederationConfiguration