The ubports installer did not help here, neither. The instruction to enable the developer mode was ok, the option for USB adb was named a little different (unfortunately I don't remember the wording), but an "OEM unlock" option was not there, and the installer could never find the device (neither on windows nor linux). The manual selection was always empty. After a while there was an error message.
Successful for me was only the git version of the magic-device-tool on ubuntu 16.04 booted from usb stick (other packages failed for me). It gives good and working instructions, it is not only a one-way tool as it also supports installing lineage os and sailfish, and it allows looking into its simple bash scripts which is very good to learn, see what it actually does and adapt for other images.
I followed these instructions: (adding the following notes)
- enable universe repository before first run of the launcher
- run "sodo ./launcher.sh" to execute it as root (if attached device is not found)
- select the "legacy" channel, which is now an alias for 15.04/stable
- after flashing and booting successful, head right into the system configuration/updates to check the channel and get the latest over-the-air (OTA) version.