Resolved:
Saving the configuration as an INI, editing the INI and restoring is the way to do it if this is your main router.
Alternatively, I have observed if you disable DHCP on the TG582n and edit/delete the LAN_private dhcp pool, the TG582n will seek out another DHCP server on the LAN to get an IP address to assign to the USB port.
I now have the surplus TG582n set up as a dedicated print server connected to my existing LAN.
ex-Plusnet (ADSL, FTTC) 2008-2023. now BT (FTTP) 2023-