Windows Admin Center not able to connect after decommissioning of old server

I provide remote support for this customer. Customer has upgraded to all Server 2019 and all workstations are Windows 10. After the upgrades Windows Admin Center was working fine until the old 2011 SBS was decomissioned. Once that happened we get the message:

"The client cannot connect to the destination specified in the request. Verify that the service on the destination is running and is accepting requests. Consult the logs and documentation for the WS-Management service running on the destination, most commonly IIS or WinRM. If the destination is the WinRM service, run the following command on the destination to analyze and configure the WinRM service: "winrm quickconfig"."

This includes on the server that WAC was installed on. We have uninstalled and re-installed WAC, we have used the "Test-NetConnection" command and it completes successfully, but we cannot get WAC to connect. When we tested the re-install we used a self-signed cert with it's 60 day limitation, but I don't think that's the cause as we cannot even connect to the server it was installed on. We did verify on workstations and server that it is listening on the proper port.

I've run through all the documentation I can find, and I can't seem to figure out why WAC will not connect to any machines on the domain in spite of the above command completing normally. I have verified that the firewall is set to allow connections on the proper port (default 5985). Any suggestions to figure out why these connections won't work before the customer has to fork out money on support through MS? They are a small non-profit, so saving them money wherever possilble.