I spent three days this week on a mountain without internet access (isn’t this a common thing in Japan?). Is it possible to restore my streak?
As far as I know, there is no way. But honestly, you were on a mountain, having a good time. That is a perfectly good reason to lose some artificial number that only exists to keep you coming back! Good for you! Focus on what matters and start a new streak. People should celebrate being off internet, not missing a day.
Unfortunately I don’t think there is a way. If you’re planning time away, even if you aren’t sure about internet access, I believe Vacation Mode freezes your streak as well as the SRS, so that would probably be the best option in the future to retain that streak.
You can mail the team, I believe that they have restored streaks in the past.