Go to Menu > My Gateways on your app, you will be redirected to Select Location.
Once there, factory reset* the gateway as instructed in How can I reset my gateway?
Then, you will have to manually remove it from your account as the factory reset procedure does not automatically do it.
To remove it, press and hold the gateway in the list (it should already be offline) until you are prompted to remove it.
NOTE: If you experience connectivity issues on your phone after resetting the gateway, your
mobile may have automatically re-connected to the gateways Access Point (Atom_XXXXXXXX).
Make sure to switch your phone to your home network so you can recover connectivity.