Garmin Forerunner 735XT

Hi. I am new on Zwift. ANd sorry for my english, I’m French…
So: I have a treadmill. Not connected. Old but it works.
I have a watch Garmin Forerunner 735XT, connected in Bluetooth on my iPhone (ios13).
I have the app Zwift on my phone, but the app finds nothing in bluetooth…So my avatar does not move :-(((((( That’s why I spend my time in the sofa to “observe” on Zwift the other riders and runners :-)))
Can anybody tell me if I can make moving my Avatar with my Garmin Watch, or not? And if yes, how ?
Or do I have to buy a Zwift Runpod ?
MERCI for your help, and Happy New Year !

2 things here.

You cannot use the Garmin as a speed source for Zwift.

The Garmin only broadcast HR in real-time and that is done over ANT+.

You could get a ZwiftPod, NPE Runn or even a Wahoo Tickr Run to start running in Zwift for iOS.

Thank’s a lot Paul !
I think I will buy a Zwift Pod
I also plan to buy a new treadmill, connected this time ! Is Zwift able to change the inclination ?

Zwift does not control speed or incline of any treadmill.