Hello tjriedl,
Make sure your third-party device is properly paired to your iPhone using these steps:
1. On your device, go to Settings > Bluetooth and turn on Bluetooth. Stay on this screen until you complete the steps to pair your accessory.
2. Place your accessory in discovery mode and wait for it to appear on your device. If you don't see it or aren't sure how to make it discoverable, check the instructions that came with your accessory or contact the manufacturer.
3. To pair, tap your accessory name when it appears onscreen. You might need to enter the PIN or passcode. If you aren't sure what your PIN or passcode is, check the documentation for your accessory.
You can see these steps listed here: Pair a third-party Bluetooth accessory with your iPhone, iPad, or iPod touch
If you device is properly paired to your iPhone and is still not working as expected, reach out directly to Garmin to see if they can offer some guidance on using their device with the iPhone. This article may be able to help with this process: Contact a third-party vendor
Best Regards.