At least some clarity is in the bugtracker.
“We do support dual SIM settings, the only thing missing is turning SIMs on/off if you have two SIMs mounted. We are aware that this is a regression from Android 6 for Fairphone 2. This feature is, however, not part of Android 6 or 7 itself, but rather a vendor-specific extension.”
And there’s the problem … the “vendor” in this is Qualcomm, and Qualcomm doesn’t support Android 7 on the (in the meantime EOLed) Snapdragon 801 chipset that powers the Fairphone 2.
So it seems Fairphone got hit by another industry modus operandi that needs tackling … too early End-Of-Lifes of integral parts preventing long-time support.
Fairphone nonetheless pulled off a Google-certified Android 7 on the Snapdragon 801, when nobody else did, but some things seem to be really tricky when they still have to use the non-open Qualcomm Android 6 drivers to operate the chipset under Android 7.
- Blame Qualcomm’s EOL policy for the regression.
- Blame Fairphone for the communication about this all, and perhaps lack of resources.
- Switch to LineageOS, if timely Android security updates are a top priority for you (also see lineageos, and be aware that LineageOS also isn’t able to switch off SIMs).
- Downgrade to Android 6, if switching off SIMs is a top priority for you.
Edit: Scrapped the Tasker reference, as someone tried this route and it didn’t work.