Step 1: Select Your Gateway
Your sensors may be showing offline but your gateway is operating.
In this case, it's very possible that your gateway filters, which are used to only allow a gateway to 'listen' for certain sensor IDs, have not been updated correctly. This can happen if a network connection had been dropped when the sensor install took place.
You can repair a gateway's filters. To do this, navigate to either the measurement point page or the gateway page in the settings menu.
Select your gateway from the list.