As it is written in the troubleshooting guide of Fairphone, in this case, perhaps you can try:
- Open your phone dial pad
- Enter
*#*#66#*#*
in the same way you are entering a phone number to make a phone call. Once the last character is introduced, the Service Menu appears showing you three sub-menus. - Select Service tests > Test Single > Touch screen .
- Use your finger to draw freely on the screen and check if you can do it in each part of the screen.
- If you can see blank areas in your drawing, your display is partly insensitive to touch.
I don’t know if it can be useful.
And of course, as @AnotherElk said: