Unable to sync old strava activities using custom picker

Under Activity Types do you have Ride, Virtual Ride, and Workout selected to sync FROM Strava?
If the problem persists it’s best to file a support request with support@xertonline.com as it sounds account related.

You may be interested in my Onboarding post for newbies –
Onboarding steps for Xert Trial users and newbies - General - Xert Community Forum (xertonline.com)