https://www.opennms.com/documentation/
https://wiki.opennms.org/wiki/FAQ-Troubleshooting
I very much doubt coming to this subreddit and asking for the command you need to do something every time you need to do something will be a winning strategy, but that's just me.
Agreed on all points, especially Nagios not being great for mapping out a Network. In the past what I've done when taking over for previous SysAdmins with poorly documented networks is install OpenNMS to map the network itself, use that to actually define Nagios hosts and write up documentation, and then take down the OpenNMS installation after.