I can understand that! I've got 2 Galaxy Watches one rose gold on 46mm Silver. The rose gold one works a charm. can send texts when the phone is off no problem, but on the Silver, I've been trying to get it to work for 11 months. Even though both watches are with the same provider on the same account, they still can't figure it out. I called @Samsung twice today and one person confirmed that you could and the other said that you couldn't.
But let me be clear, I own a Samsung Galaxy Watch 42mm that DOES send and recieve text messages when the phone is turned off or in airplane mode.