There is.
The reset that I presume you have performed, i.e.
Settings>System>Reset options>Erase all data
does exactly that. It doesn’t reinstall the OS.
To go further, you can perform a “clean install”. This would require use of a computer, it would require unlocking the phone’s bootloader and consequently would delete all your data on the phone.
For more information see the article How to manually install Android on your phone from Support at Fairphone.
If you are not confident with such work, you might be able to find someone who could give you more advice. Have a look at the map of the Fairphone Angels.