After upgrading to Fairphone OS 18.09.2 I discovered I could no longer switch off SIM’s. So I decided to revert back to Fairphone OS 18.04.01. While doing so I ran into a problem: after reinstalling Fairphone OS 18.04.01 the phone wouldn’t boot completely anymore, it kept rebooting constantly. However, after also doing a factory reset the phone worked fine again, running Fairphone OS 18.04.01. My tip therefore would be to follow up the reinstallation of Fairphone OS 18.04.01 with a factory reset.
Performing a factory reset naturally causes you to lose all data, so it is paramount to make a full backup while still running Android 7.
To avoid losing data, the detailed order of things to do would be then:
Perform a full backup. Instructions on Fairphone site: https://support.fairphone.com/hc/en-us/articles/210512123
Download and Unzip image file https://storage.googleapis.com/fairphone-updates/7058d8ad-5694-4a1b-95c3-db9a76218c49/FP2-gms-18.04.1-manual.zip
Follow instructions 1 through 8 on this page: https://support.fairphone.com/hc/en-us/articles/207914363-Manually-install-Fairphone-OS-for-the-Fairphone-2
If you’re lucky, you’re done now, and you’ll even still have all of your apps and data.
If you’re not, and you’re stuck with a continuously rebooting phone:
Remove the battery to force it to a full stop.
Carefully read the instructions under “If your Fairphone 2 doesn’t start anymore” on this page to do a factory reset: https://support.fairphone.com/hc/en-us/articles/115001610926-How-to-return-to-Factory-settings-on-my-Fairphone-2
Insert the battery and immediately initiate the factory reset (you have to quickly boot into system recovery mode before the phone starts its own perpetual rebooting loop).
After performing the factory reset your phone flawlessly runs Android 6 Marshmallow again. Unfortunately, you’ll have to reinstall all of your data and apps again. But that is a small price to pay for having a well working phone again.