Issue with activity monitor and watchdog service for servers across different domains

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as subject, FAM core server is in Domain A, while one of file servers is in Domain B, this is a new node to deploy activity monitor/watchdog services.

installation for activity monitor went through, a new self-signed cert for AM is in place on the new node

installation failed for watchdog, it complained about certifiate as unable to establish trust for SSL/TLS secure channel with core server at port 8000, it had watchdog self-signed cert installed, then got deleted after watchdog installation failed . 

I am seeking help here, any input will be much appreciated!

Could not establish trust relationship for the SSL/TLS secure channel with authority 'mydomain.domain.com:8000'. ---> System.Net.WebException: The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel. ---> System.Security.Authentication.AuthenticationException: The remote certificate is invalid according to the validation procedure.