You can technically configure an opensim server in such a way that you can connect locally AND remotely behind a loopback router... but it... is not ideal. If you configure the User Server (Robust) to run on a domain name (or free subdomain) and your region to run on static IPs, it works.
The local computer can connect by adding the server's lan ip to the grid manager in Singularity (This trick doesnt work in Firestorm).
The host computer must have the server's public IP in it's hosts file.
This configuration causes 3 major problems:
-You MUST use singularity.
-It breaks hypergrid connections (you cant return after teleporting out)
-Firefox has severe issues with reading and editing parcel information including windlight settings.
You will recieve an error in the console reading
"[LAND MANAGEMENT MODULE]: Unable to find parcelID -1"