How do I connect my Vivoactive4 to a new iPhone when my old iPhone is broken?

Your old data will always be there as long as you login to Garmin Connect with the same credentials you've always used. Do NOT create a new account.

• On the old phone remove the vivoactive 4 from Garmin Connect under the Menu, then Garmin Devices.
• On the old phone under Bluetooth settings "forget" the vivoactive 4.
• Put the vivoactive 4 in pairing mode - Hold back button (bottom right), select gear icon, Pair Phone
• On the new phone pair the vivoactive 4 using the Garmin Connect app. vívoactive 4/4S - Pairing Your Smartphone

The screen on my old phone is broken, so I can’t access the menu to follow step one above.

Do you plan on using the old phone again? I would just power it off, then continue with step 3.