I like the Wilson brand of amps, I have a Wilson Sleek and am amazed how well it works. I have used it inside buildings at work and out in the boonies. They have cradle based amps that will work for a single phone or hotspot, they are cheaper and more portable. They also have hotspots that will allow multiple devices to connect, they cost more but allow more flexibility with a permanent install. Depending on what type and if you want 3G to LTE you can find one from about $100 to $500. The situation you describe sounds ideal for an amp, having a signal but one that is not strong enough to be practical.