So, looking up the Viper site for your county, it's all running at 852MHz range.
RG-58 coaxial cable is extremely lossy on those frequencies. Depending on how long your coax run is, that could be the issue.
If you coax run was 50 feet (example), you'd lose 80% of your signal just in cable losses. If your cable run is shorter, the losses are going to be lower, but likely still significant.
Foliage can attenuate the signal a little bit, but usually not enough to make that much difference. Local topology between you and the tower site will make a bigger impact. If it's a clear line of sight between your antenna and the tower (ignoring the trees), then it should work with good coaxial cable. If there is a hill/mountain in the way, then you might have issues.
So, the big question now is, how long is your coaxial cable? Upgrading to better cable -might- be the answer.
Antenna type can impact things a bit, if it's not designed for use at 800MHz, but I'm going to think the coax is the bigger issue.