When I click on devices it says "You have no device on Garmin Connect. Add a device using Garmin Express." but on Garmin Express it clearly shows my Forerunner 910XT".
Do you, or did you previously, have ANT Agent installed? If so, uninstall the ANT Agent software. Then open up your AppData folder (C:\Users\<username>\AppData\Garmin if I remember correctly) and rename it. Now remove the problematic device from Garmin Express and then re-pair it.