Some facts... And some ideas.
First: Boosters like Max Signal and We-Boost do work. if there is any cell signal at all they will make it stronger.> BUT.. NOT ALL OF THEM will increase Data speeds. Some of them only work at 2g or 3g Speeds not 4Glte or the new 5g that is coming out... So make sure you get the right booster. NO I CAN NOT HELP
Second.. as others have said if the tower is busy.. bandwidth suffers. also your account may throttle based on usage.. Mine does even if it's "Unlimited"
ANd of course no company has 100% coverage save for the Sat-Com outfits. ANd they are both expensive and generally slow.
New Sat-Com expected soon but I dont' know how soon so will say nor more.
Best option is multiple accounts say Verizon and AT&T or T-Mobile/Sprint/Virgin That gives you the best chance at a local tower.
Now.. Jet-packs and My-Fi devices...
An interesting thought is to take a old DirecTV dish (Round dish) and mount the device where the Horn is. then log into it and look at signal strength Should be a high gain antenna (Give me frequency and dish diamater and I have a program that estimates gain.
Also HEIGHT is might.. I know one camper who mounts it atop about 20 feet of telescopign fishing pole alongside his rig for a much stronger signal.
If you use a booster I'd put the outdoor antenna on some kind of telescoping mast. You can also get directional antennas that have higher gain to put atop the telescoping mast. So far I'm not needing that but. it helps.
I do use a telescoping mast with a Umbiquiti Nano Station Loco-M-2 for park Wi-Fi.. That puppy is great.. They make some even greater.