Are you using the Vol+ from the keyboard or the toggle switch on the CoDi? I think it has to be the one on the CoDi.
I also recall it's a bit fiddly with the Esc and Vol+ to get from the dead android screen to the recovery menu proper. I think the sequence is:
1. Press (and hold) Esc
2. Press (and hold) CoDi Vol+
3. Release CoDi Vol+
4. Release Esc
I'm not sure on this sequence, it's been a while, but I remember it being a bit fiddly and particular.
If you are getting a reboot the device may be only seeing your Esc key press which is also the power button.
Here's the link to the Planet Linux procedure anyway, this has worked for me in the past, let us know how you get on.
https://support.planetcom.co.uk/index.php/Linux_for_Cosmo