Unfortunately imo the Fairphone team made a very bad design choice and thus the Fairphone 2 has no physical back and home buttons. They just waste 1 cm of space at the screen bottom of the phone.

For FP2 to be usable, i.e. one-handed usage, Ubuntu Touch therefore needs to provide a gesture for "back", analog to what the iPhone X has for example. Or let me enable in the settings Android like buttons on the bottom of the screen.

I also filed a feature request here:
https://github.com/ubports/ubuntu-touch/issues/592