The Frontier X uses Bluetooth Low Energy Technology (BLE) to sync with other devices like:
Apple Watches, Garmins, Whoop Straps, Pelotons, Zwifts and more as a heart rate monitor.
If your device is equipped with Bluetooth Low Energy Technology, the Frontier X2 should easily sync as a heart rate monitor (HRM) with no issues whatsoever. The Frontier X2 can simultaneously connect to a maximum of 3 devices - 1 Mobile App & 2 External HRMs.
While the Garmin device will only reflect Heart Rate, the Frontier X2 itself will reflect various metrics such as ECG (live and recorded), Breathing Rate, Strain, Heart Rate, Body Shock, and Training Load once it is connected to the Frontier X2 app on your phone, post your workout. You can see a more detailed view on the Frontier X web dashboard, once you've uploaded data from your device to the cloud.
Here's a guide explaining the steps to connecting your Frontier X to an external device: https://fourthfrontier.tawk.help/article/15-connecting-the-frontier-x-to-an-external-device
The video uses a Garmin watch as an example, but the steps across all devices is broadly the same.