I don't understand how this does not help you with the Charge 2 in the phone app. Please read again carefully, paying special attention to the parts I have put in bold and underlined.
How do I set an alarm on my Fitbit device?
For Fitbit Ionic and Fitbit Versa series, set alarms on your device. For instructions, see On-Device.
For all other Fitbit devices, set alarms in the Fitbit app. For instructions, see Fitbit App.