@Lonewolf1038 

Hello,    Client devices listed under "Offline Devices" means that the dev ice is no longer connecting to your Wi-Fi on the Deco Mesh.  The simple fact that it is listed under "Offline Devices", also means you are correct and that at one time it did connect. 

The Offline device will eventually be purged from that list, if the device has not re-connected after a period of time (not sure just exactly what that time period is).

I would work on getting the Fitbit reconnected to your Deco Wi-Fi Mesh, from the device side. Normally getting a device to re-connect involves forgetting or removing the current Wi-Fi network SSID name, (your Deco Mesh Wi-Fi), from the Wi-Fi list on the device in question. In this case your Fitbit. 

I did a quick search on Google and found the following; " Why won't my Fitbit connect to my wifi?

Restart your watch by pressing and holding the back (left) and bottom buttons until you see the Fitbit logo. Go back to the Fitbit app on your phone or tablet and try again to add a Wi-Fi network. If you see other networks appear in the list of available networks, but not your preferred network, move your watch closer to your router.

If that does not work you can try the following link;  https://help.fitbit.com/articles/en_US/Help_article/2233.htm

If none of that helps, you may want to reach out to Fitbit support. 

I hope that this helps.