You shall be able to change the user name from command line. See https://docs.nethserver.org/projects/nethserver-devel/en/v7/nethserver-nut.html#slave-node
I cannot test it, but with the outcome of these commands I might be able to help:
config show nut-server
config show nut-monitor
Another option would be the new cockpit UI in development:
Edit: It should suffice with these commands (to set the user):
config setprop nut-server User qnapups
signal-event nethserver-nut-save