We back up servers on different networks in our data center. Make sure the necessary ports are open through whatever firewalls are implemeted. If they can talk they can be backed up. Configuration will depend on the backup software you use.
Offering service particularly in backup files business are very critical. The best thing you have to do is to ask IT experts or even hire them to do those stuff. This is because you or your company might be sued for a legal case if you have not accomplished the backup tasks.