Hi there @NickGleeson.
Thanks for the details shared on your post and for the troubleshooting tried so far. Usually the Fitbit Ionic is able to respond if you follow the instructions listed at How do I use the device lock on my Fitbit device? However since you have mentioned that your phone does not show the device, I recommend doing the following:
Important: Please note that you'll need to reinstall any apps on your device after the reset, and if you couldn’t sync your device, then any data on the Fitbit device will be lost.
On the device, hold all three buttons for about 10 to 12 seconds.
When the Fitbit logo disappears, quickly release the bottom right button, continuing to hold down the left and top right buttons (releasing the bottom right button as soon as the logo disappears is time sensitive).
When the device vibrates, release all buttons.
Wait for approximately 1-2 minutes while the device clears all data. Eventually, the Fitbit logo appears and the device boots and displays fitbit.com/global/us/setup
When the device successfully resets, please set up your Ionic as new device. See How do I set up my Fitbit device?
Hope this helps.
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