Unable to contact the source site – Migration Fail

While using this plugin to migrate a site that’s behind a VPN, the plugin gets stuck on the, “Verifying Details…” page and times out after ~10 minutes with the following error: “We are unable to contact the source site. Due to this the migration cannot be completed.”

I also received the following console error: “Failed to load resource the server responded with a status of 504 (gateway timeout)”

Could this be a firewall issue on the source-side? If so, what should the client allow on their firewall to get this process to work? Thanks