We had earlier been using Observium in my previous company.
Wanted to use LibreNMS VM for monitoring a Demo Setup in a client’s production environ to monitor HP Servers/Storage, but the customer does not allow Internet access from the server or from inside the Data Center.
For the server part you’ll need to copy the rpm files over you need.
For LibreNMS, you can install it on another server but don’t create config.php or run the actual install.php script in a browser, then zip up /opt/librenms/ and copy it to the new server. Run install.php from a browser and finish the install off. Should work.
You should then recommend to your client to at least have a proxy server infront of this box which you can allow access to github.com so you can do updates easier. Allowing to yum repos would also help a lot.