Welcome to Justanswer.com, my name is Dustin.I'm sorry to hear about the issue you are having, I will do my best to help you resolve it.
Since the device was previously paired with another computer.. Most likely, we need to reset the device to allow it to pair with your current phone and computer.
This video will show you how.
After its reset.. You can try to pair and connect it with garmin connect again.. This video will show you how.
Though, you may need to download and install a new version of the garmin ant agent software to allow the connection.
If you have any further questions or concerns, or run into any issues, let me know.
And if my steps help you to resolve the issue, please dont forget to return and click and submit a 5 star rating so I am credited for successfully assisting you. Thank you,
Question.. Did you attempt to install garmin connect as I suggested?