Unless you installed the “old interface”, the URL to connect to the NS web interface is https://IP:9090
I would suggest statically setting your PC IP address to the subnet that you assigned your LAN interface on NS and then attempt to gain access to it that way.
Without knowing how you setup the interfaces on install it would be hard to know for sure what is going on, but I do know that during setup I had to set both network interfaces to AUTOSTART and then on the general tab click “automatically connect to this network when available" or they wouldn’t both come up on first boot.
I did write up a little migration guide for when I swapped over from PFSense, it might help for your Sophos migration.