We only get one bar and the signal range is between 83 and 88, how can we get a better signal on the RF825 with a sprint account?

The only option to improve signal strength on the sprint is to use different Antenna. There is nothing you can change in the settings or firmware to improve signal strength.

Here are some Antenna suppliers that we work with:

http://www.wilsonelectronics.com/

http://www.mobilemark.com/communication-antennas-leading-antenna-supplier-mobile-mark.html

http://www.nearson.com/