My wifes cover came off of her watch as well.
The steps I took:
1. clean up any excess goop from the old adhesive on both pieces.
2. Get 3M 2-part epoxy adhesive 2216
3. apply very small bead around opening edge.
4. place sensor cover in place. Be sure to align clear areas with the led sensor.
5. gently press in place. and wipe excess off without getting it on the clear areas of the lens.
6. allow epoxy to cure.
In my case Samsung moderator invited me to private chat, asked me for receipt and serial number, and eventually denied coverage. I gave up on Samsung gear after I learned so many people had issues and warranty was not good
Thank you for reaching out. Just as a heads up any self-repair of your device would void the warranty on your device.