You might try a signal booster like this
https://www.wilsonamplifiers.com/web...er-kit-470103/
Basically you use a very directional antenna to find a good spot outside and the signal gets amplified and re-distributed inside your house. When I 1st moved here I had very marginal signal and one of these got my calls to work and data to flow pretty consistently. My external antenna is mounted about 12 feet up a tree and pointed at a cell tower in Laconia (line of sight, but 12 miles away).
The manufacturer used to accept returns if it didn't work for you (and I suspect they still do since I see refurb units in their "specials" page). You can also get it on Amazon. These get pretty good reviews on Amazon.