Salutations everyone, not sure if this is the right category for this but its network hardware related so guess I'll ask. I'm attempting to figure out what gain antenna I would need for a cellphone booster to get a more reliable/consistent signal with my unlimited hotspot internet. I know "why are you using hotspot internet", because I live out in the sticks and can't get the 500 mbs internet that's in the town I live only a mile outside of yay crap geographical location and its better than the WAN companies that hawk their internet around here they are all super overloaded so when I had it it would get worse than dialup. Anyway, the tower in question is a rather large cell tower I'm not exactly sure how tall but it is tall enough to not be obstructed by any trees or any hills in the area and its located about 6 miles away from me. It's weird my phone gets pretty good service at my house but my hotspot is consistently losing signal and losing dns meh the antenna in it is probably garbage (I'm getting a replacement so I'll be able to rule out if its the hotspot itself causing the problem). I was considering mounting the booster antenna on my roof at about on a pole that's about 3.5 to 6 ft. My question is how much dbi gain on the antenna would I need for that? I already know I'm going with a directional antenna. I appericate any help and thanks for reading. 
Andrew McNeil on YouTube has lots of videos on antenna theory and practical application. Directional designs like a yagi or biquad seem to work very well if you can get it aimed properly. Some of the ones you find on Ebay, Amazon, and such have flaws to them which ruin their operation. My understanding is that many of them list the theoretical gain of the design and no actual testing is done.
I don't know much about cell phones or the frequency they run at, but some of the old videos there where he buys an antenna, tests it, goes over design flaws, and attempts to fix it are pretty interesting if you like that kind of stuff. It can help you keep an eye out on what to avoid or what might be worth buying and fixing.
I guess my main point here is that some designs are good, but their implementation is poor. You want as high of a gain as possible, and given this will be in a fixed location it should be straightforward to aim the antenna and leave it in place. A high gain directional antenna has a very narrow beam width, but once properly aimed it would give you a great signal.
I would check out the Ham radio forums. The good news is the target is only 6 miles away and not moving.
I wouldn't so much worry about gain but where you mount this antenna externally. Line of sight is a more important factor (even at these frequencies) than signal amplifying. I would experiment with your signal booster outside closer to where you would want to mount an antenna for the device to rule out doubts in your equipment or theory. If it looks promising, choose a directional antenna that is designed for the frequencies that your provider is using and mount it towards and in line of sight of your tower. profit.