Bontrager Speed and Cadence sensors work as expected. I've used them with two different PC7s.
https://www.trekbikes.com/us/en_US/e...nsors/p/09814/
I bought mine at a local shop that carries Trek bikes.
What Garmin sensor do you have?
You might have to plug the head unit into your computer and fool around with the settings and make sure that it's not looking for a SRM crank (that also serves as the cadence sensor).
EDIT:
It's not a well-known feature, but it works. Even the SRM sales rep didn't know that it would work when I mentioned it on the phone.
Attached is a pic of the setup on my Snyder. You can barely see the PC7 on the handlebars (blue). You can see the ANT+ cadence sensor on the left chainstay. You can't see the speed sensor on the left fork.