How can I get better cellular signal to my GSM panel?

The GSM Alarm System main panel includes ports on the BUS panel on the back which allow you to connect a more powerful antenna to the panel. This will often increase the effectiveness of the system in picking up signal from nearby cellular towers.

Ports 15 and 14 (ANT and G_ANT) are for an antenna and a ground cable, respectively. You can use a 6.24-inch length of copper wire (900 MHz, half-wavelength) to make an antenna, and connect it to Port 15. Make sure the length of the copper wire is as close to exact as possible.

Below, for your reference, is a picture of the BUS bar on the back of the main panel.