Hello,
Activities in the app will not open, app has been deleted, phone reatarted and app reinstalled. Any suggestions? Thanks and kind regards John
Do you not see your activities or are you just unable to open them to view the details? What do you get when you tap on your profile icon.

then go to the Activity tab? You should have your most recent activities listed. Then you can tap on one of those to open it.

If you cannot see any activities, it could be you've logged in with a different Garmin account, as mentioned here: Data is Missing from My Garmin Connect Account After Syncing
Can you post a screenshot?