Did you check the TLS version(s) your PowerShell session can use?
[Net.ServicePointManager]::SecurityProtocol
If TLS12 is not in there, run the following before trying the connection
[System.Net.ServicePointManager]::SecurityProtocol
= [System.Net.SecurityProtocolType]::Tls12
, [System.Net.SecurityProtocolType]::Tls11,
[System.Net.SecurityProtocolType]::Tls