I'm currently locked onto AT&T Band 14 for my MOFI modem. My current signal strength looks like this:

Here are the results of my band scan:

I currently have the MOFI sitting on a window sill, using this antenna on the outside of the window.
https://www.amazon.com/gp/product/B00DN ... UTF8&psc=1

Without the antenna, I can only lock onto the slower 700mhz bands.
Speedwise, the best I see is just under 30 Mbps. The tower is about 1.5 miles from my house, but the line of sight is mostly through trees.
My theory is that with a better antenna, I can improve the signal strength and get better speeds. Also, once the leaves come back in, I won't see as much of a performance drop.
So, what is the suggested antenna setup for my situation?
Thanks in advance.