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. Find instructions on the Fairphone site: https://support.fairphone.com/hc/en-us/articles/210512123
On your computer: download Android 6 image file from here: https://storage.googleapis.com/fairphone-updates/7058d8ad-5694-4a1b-95c3-db9a76218c49/FP2-gms-18.04.1-manual.zip
On your computer: unzip the downloaded image file to a temporary folder.
Follow instructions 3 through 8 on this page: https://support.fairphone.com/hc/en-us/articles/207914363-Manually-install-Fairphone-OS-for-the-Fairphone-2 (skip instructions 1 and 2, as those instructions would make you download the Android 7 image file again).
First, carefully read instructions 3 through 8, esp. instruction 4. Best: print them, and keep them at hand as you proceed. Then follow the instructions precisely and completely.
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 (like I was), you’ll have to perform a factory reset:
Remove the battery to force the phone to a full stop.
First carefully read the instructions under “If your Fairphone 2 doesn’t start anymore” on this page: https://support.fairphone.com/hc/en-us/articles/115001610926-How-to-return-to-Factory-settings-on-my-Fairphone-2
Note: as you are restoring Android 6, you have to follow instructions 1 through 4, SKIP 5, and jump to instruction 6 on that page.
To start performing the instructions in step 6, insert the battery into your phone and IMMEDIATELY initiate system recovery mode (instruction 2 on the page in step 6)
Note: if you don’t boot quickly into system recovery mode, the phone starts its own perpetual rebooting loop; if that happens, go back to step 5 (remove battery) and retry).
Note: ‘system recovery mode’ is NOT the same as ‘fastboot mode’ which you used while reinstalling Android 6, carefully read the instructions!
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, but that is a small price to pay for having a well working phone again.