Hi Although my phone is an A5 (2017) I had calendar sync problems after last security update (up to then no issues). My phone syncs to Gmail but not Outlook calendars. I found that sync does not occur if using data bundle connection i.e. 4G but DOES sync if changing phone to WiFi connection (say to home router)!!! I have advised Samsung Tech and hope they can fix issue. Maybe this could help you too??
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