You said you already installed Ubuntu Touch.
I'm presuming that you have unlocked your bootloader before doing so and that you might have used the terminal and might be running Ubuntu.
If this is correct it should be no problem for you to:
- With your device powered off, press and hold the volume down button and then the power button simultaneously until the screen turns on to enter "Fastboot mode"
- Connect your device to your PC
- Open a terminal window and type
if that does return
no permissions fastboot
it can be fixed by running fastboot as root from now on, i.e.
sudo fastboot [...]
- next, enter
fastboot oem device-info
if the output contains the line
(bootloader) Device unlocked: true
you are able to flash a custom recovery.
- To flash the recovery go directly dl.twrp.me/mako, and download the latest (topmost) image.
Now, find out where the image is downloaded and open a terminal there.
Then enter this command
fastboot flash recovery twrp-3.3.1-0-mako.img
(again with sudo if necessary)
- Reboot and you should be good to go