MacBook Messages won't sync

I have a MacBook Air since last October, and it used to sync with my iMessage app on my iPhone 6S.  However, for the past several weeks, messages I send through the Messages app on my MacBook don't sync or show up in my iMessage history, and vice versa.  Prior to 2 weeks ago, FaceTime was also affected, but I was able to get both FaceTime and Messages functioning properly with the help of Apple Support via phone.  However, the issue is back with my messages.

MacBook Air, macOS High Sierra (10.13.3)
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Problem solved, thanks to Apple Support.

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