Vivo Smart HR won't track sleep


I have an older version of the Garmin Vivosmart HR Activity and it's been fine for what I need.

I lost my charge recently and after I ordered another one I noticed that the sleep monitor has dropped off the phone app as being monitored.

I expect this is something simple.

How do I get that details back on to my app?

Everything else is syncing ok - steps, heart rate, calories etc