Talk to your provider, explain to them you get poor service at your home or place of work, and they'll send you a free Internet-in cellular-out radio AP. She doesn't need a tower-based booster if she's got fiber/cable/DSL, those only serve to amplify weak signals and she's too many miles and too many mountain ridges away from the nearest tower, she wants something with RJ-45 input, a little GPS antenna so the cell supports e911 location data, and it will broadcast LTE (or now 5g) cellular data.
I work at a shop with metal walls located in a river valley. It's a cellular data black hole. People used to climb the hill up the driveway to make and take calls, but various people called their ATT, Verizon, and T-Mobile providers and all three shipped us femtocells. Mow the users and the contractors/customers who come to visit can't even tell that their phones have switched to data over our ISP instead of a tower, it just works - including 2FA codes and MVNOs.
She may have to switch to first-party Verizon service instead of using an MVNO.