What I’ve been doing in the past is view the Xert workouts in a table, compare the dates to the Garmin Connect activity list, note the missing dates on Garmin, check missing workouts on the Xert list, then click download FIT file button in Xert, then drag the missing workouts to the Garmin import box.
However, since the recent xertonline update the button to download as FIT file is now missing. It is supposed to be added back soon –
I don’t think there is an easy way to automate this process from Xert since Garmin doesn’t support it.