QuadPro's highly directional gain can help improve signal up to 20 miles from the tower, and will work even if there are trees, buildings or other obstacles between you and the tower. QuadMini is an omnidirectional antenna and benefits from being a little bit closer to the tower, but it can improve signal quite far from the tower.
The exact range of the antenna depends on the bands you're connecting on - the 20 mile number above assumes you're connecting on the 600 MHz n71 band that T-Mobile uses for broad coverage, but the maximum distance is lower for higher frequency bands.
That being said: we don't recommend buying our antennas if you have no signal at all outdoors. If you have to drive away from your location to get T-Mobile coverage, an external antenna probably isn't a good fit, but feel free to reach out so we can help you.