Garmin devices connect via ANT+. Just about every other piece of technology connects wirelessly via Bluetooth instead. The two technologies are not compatible.
It is possible to use a bridging device such as the North Pole engineering CALBE, or the Viiiia Heart Rate Strap. These devices convert ANT+ to Bluetooth.
Alternatively, if you have a Wahoo KICKR or another trainer that sends power but not cadence, the Wahoo TICKR X is an excellent choice.