I was unsuccessful getting a new site connected to the dashboard.
The client happened to tell me he had called SiteLock on an SSL matter.
(they are not hosted with us)
I realized SiteLock was probably blocking the connection.
We got on the phone with them and yes,
their firewall was in fact blocking the incoming connection from MainWP.
They whitelisted the ip address and the dashboard was then able to connect.
A SiteLock question will get added to our onboarding flow.
When we get large enough, we will begin requiring new sites be hosted with us.