The Flex2 is end of life and it will need to be updated before use. You will need an older version of the Fitbit application to upgrade. Once upgraded it will work on the latest IOS and Fitbit app. I used iPhone 5c (IOS 10.x). Upgrade pebble and it works on iPhone 11.
Specifications:
Activities Tracked: Total Steps, Daily Workout Memory, Daily Distance, Calories BurnedBattery Type: Lithium Polymer
Device Material: Silicone
Supported OS: Android, IOS,Windows, MAC
Wired Connectivity: USB
Wireless Technology: Bluetooth
Water Resistant: Device is water resistant
Operating Temperature: -4° to 113° F (-20° to 45° C)
Non-operating Temperature: -22° to 140° F (-30° to 60° C)
Small 5.5"–7.1" 140 mm–180 mm
Large 7.1"–8.7" 180 mm–220 mm
Features：
Flex tracks steps, distance, calories burned and active minutes. showing progress with LED lights.
Device monitors your sleep with Automatic Sleep Detection.
Wakes you with a silent or vibrating alarm.
Set goals, view progress and earn badges as encouragement.
Rain, splash and sweat-proof device ensures durability.
Rechargeable battery lasts up to 5 days on a full charge.
Bluetooth 4.0 automatically syncs data to your computer or other device.
Adjustable band makes the perfect fit easy.
Package Include:
1 x Fitbit Flex 2 Tracker
2 x Classic Wristband/strap (size S and L )
1x Charging cable