Filter may or may not help, depending upon severity of Cell phone signal strength at your location. You can find this only by actually inserting/removing filter and observing change on performance.
Meanwhile you can easily conduct an RF scan using your Pi, dongle, and anten a by method given in this thread:
Do I Need A Filter?
You can also determine what maximum range you can get by method given in this thread:
What is the Maximum Range I can Get?