I'm beginning to think that this RUT240 might be a bit unwell.
I've tried upgrading the firmware several times ("From file" because the internet connection is too unreliable to perform an "Update from server") but it never succeeds. Once it got as far as verifying the .bin file and gave me the final choice to upgrade or abort, but after a bit of waiting the router became unresponsive then reappeared with the old FW version.
I tried the CLI "firstboot" option but nothing happened - I was expecting to see a message like "This will erase all settings and remove any installed packages. Are you sure? [N/y]".
This page suggests that the CLI "firstboot" performs the same actions as the hardware pin reset and the WebUI "Restore default settings" so maybe there's not much to be gained from investigating the CLI failure. https://wiki.teltonika-networks.com/view/RUT240_Device_Recovery_Options
I'll keep trying but if there's anything else you can add I'd be very grateful.