I can tell you that I've been chasing this same issue with Facer for a while now.
I have an iPhone 7+ paired to a Huawei Classic1, TicWatch2, Apple Watch Gen1 and Gear S3.
I also have Galaxy S7 phone paired to another Gear S3, S2, LG Urbane 2nd Edition. So, I've worked with lots of phone/watch combinations.
The core issue is that the Facer for IOS app (phone app) does not have an account login. So, even if your Facer Creator is logged in to your account and you select "Send to Watch", the phone app is not connected to that account and the face cannot be sent to your watch.
Coincidentally, I pinged Facer again this week to at least comment on whether or not an iOS account connection is in the works. I know that Apple does allow this type of connectivity as I've accomplished it with other iOS apps.
For now, iOS users are unable to purchase any Facer premium faces for either WearOS or Tizen watches paired to an iPhone.
Here's a workaround idea that is not ideal, but might at least allow you to test the face.
Publish the face as a FREE face. Immediately sync the face to your watch, which you should be able to do from your iPhone. Then delete the published face. Once you finish testing, you can re-publish as a premium face. Just a thought.
Good luck with your efforts. I really enjoy your work prior to becoming a Premium developer and I'll bet that your new interactive work will be awesome.
Facer is a great platform, but this is the one point of frustration.