Communication between iPhones and Samsung Gear watches is still a bit of a mystery. When you hit the right combination of events it syncs beautifully. Other times I bang my head against the wall trying to reproduce the setup. I have a variety of watches and have Gear S3 as well as S2 and Galaxy Watch in addition to WearOS and TicWatch). I have Facer for IOS Premium. It's worth the money! By paying for the premium application the watch can see syncs coming from any device logged in to your Facer account. I'm able to send faces from my Macbook, Windows laptop, iPad and iPhone. Additionally, I design faces and can send drafts from Creator to my watch.
So the issue... there are some tips that Facer offers on the Community site (I'll see if I can locate the link.)
What I find is this... Make sure that the watch is connected to WiFi. With both Gear App running on the iPhone as well as Facer App, I QUIT the Gear App on the iPhone. This forces the watch into "Standalone" mode. When the stars are aligned, this scenario allows me to synch anything I want to the watch from phone or other devices logged in to my account. Sometimes this doesn't work. I try a variety of steps including restarting the watch, restarting the iPhone, quitting and restarting Gear and Facer apps on the phone, etc.
I wish I knew a reliable and reproducible method to share with you. Be patient. it will be worth it.