If the cellular signal for the internet back up is poor you will need to extend the cellular antennae.
For runs of <15ft, the Panorama Antenna B4BE-7-5SP (http://www.panorama-antennas.com/site/index.php?route=product/product&product_id=67) is the recommended antenna.
For longer runs, we recommend the Cradlepoint 170668-000 Multi-Band Omni-Directional Antenna (https://www.jefatech.com/products/170668-000). This will require a specialed cable to attach but can support runs of up to 200ft. The needed cable is CradlePoint to External Antenna Cable Assembly - SMA Male to N Male (https://www.jefatech.com/products/ca-400f-nm-sma-xxx).
Each router has two antennae so you need to purchasetwo.