The Frontier X uses Bluetooth Low Energy (BLE) Technology/Bluetooth Smart to sync with other devices as a heart rate monitor. These devices include certain models of Apple Watches, Garmins, Whoop Straps, Pelotons, Zwifts and more. If your external device is equipped with BLE/Bluetooth Smart 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 external 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 mobile app, post your workout. You can see a more detailed view on the Frontier X web dashboard as well, once you've uploaded data from your device to the cloud.
Please note that the Frontier X2 does not support ANT+