The reason I mention the above is because you mention as well " Installed an 8dBm gain antenna". Normally, antenna gain specs are stated in dB, which is a signal strength ratio, and not an absolure measure, as explained in this Wikipedia article:
Antenna gain specs as stated in Decibels are a measure of ratio, in other words the increase in signal amplitude you can expect when using the antenna. So if you are thinking of a Yagi antenna, it should help, but do remember that the signal gain of the antenna is achieved through directionality, The antenna is designed to be more sensitive to RF signals in one direction, while much less sensitive in all other directions. Most antennas will have a polar graph published for them that illustrates this for that particular antenna.
The result is, that when you use a directional antenna, it is critical that you point the antenna at the cell tower with some care to take advantage of the antenna's directional characteristics.
The Wikipedia articles referenced above also give info on what those numbers mean in practical quantities. For example, a 3 dB difference in signal strength, depending on which direction, is a signal either 2 times as strong, or 1/2 as strong.