By default Duplicati only binds to the localhost IP address, so it’s not accessible from another computer. Setting the “remote access” option causes it to bind to all network interfaces. If you do that you can access Duplicati by going to http://remote-machine-lan-ip:8200 - no need for a reverse proxy at all.
Settings in Duplicati covers password, remote access, and hostnames (the one for this server) setup.
I did this and tried to restore, it goes to listing backup dates before failing to connect