Email Setup Configuration in Google Cloud Compute VM Using Mailgun

I installed mailgun via cli. But something must have gone wrong during the installation. Even though one domain is working properly the other domains in the project have stopped working. I need someone who clearly understands how to use the Cloud Console, has knowledge of SMTP port 25 issues and firewall/api solutions to these issues. The outcome will be working email for sending and receiving to and from the server. WHM/CPanel are installed. Mailgun was installed via cloud console. If you understand the project, I would like you help to resolve the email issues on the server. -Many thanks!