Since Nougat update I can't catch Pokemon anymore

Sorry, but this is wrong.
As long as 18.04.1 is still offered on the page along with this guide …

https://support.fairphone.com/hc/en-us/articles/207914363-Manually-install-Fairphone-OS-for-the-Fairphone-2

… this guide is totally correct and to be followed in detail.

A guide with a direct download link to 18.04.1 (in case the download on the support page switches to 18.09.2) is here (“I regret upgrading, how do I downgrade?”) …

You are instead advising to use an incremental 18.03.1 → 18.04.1 OTA update file intended for use with the Updater or the recovery … with the manual fastboot installation procedure for installing the whole OS … Good luck with that, indeed. It will not work.