some of my activities between the end of 2016 and 12/31/2019 disappeared from my statistics, despite being the same watch and the same user ... Can you help me?

If you're sure you have never had more then one Garmin account, verify the activities exist by checking Garmin Connect Web. That should at least give you some comfort that they are not gone. Then we would know if the issue is with your account or with the mobile iOS app.