I have a Fusion Bio and I can't use it with my iPhone 7... I used the last version of the Striiv app, but I can't connect with my tracker. It was working OK with my iPhone 5S.
Is the iPhone 5s still in operation? If so, that phone's Bluetooth may still be holding onto the pairing with the Bio. In that case, go into the iPhone 5s's Bluetooth settings, find the Bio in the list, then tap Info > Forget This Device.
Give this a try and let me know how it goes.
Striiv Customer Care