I have been looking for a way to save activities on my phone without using an internet connection. Over the years there have been quite a few inquiries here on the forum about this, so I thought I'd share this. I stumbled across Gadgetbridge, an Android app that saves activities in FIT format on the phone. It can also store and display sleep, HR, steps data etc. The data is only stored on the phone and not uploaded anywhere. It works perfectly with my Forerunner 955. Gadgetbridge works with many more devices, I tested with my Colmi R12 smart ring and that works too. This is not an advert and I am not affiliated with them, I am just very happy to have found this, and hope it helps others.