She just needs a microcell/femtocell.
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.
> She just needs a microcell/femtocell.
Those come with their own set of problems. In particular, they have to be able to receive a GPS signal, which is often not possible in mountainous terrain. I had a microcell for years and it was nightmarishly unreliable. Not only would it regularly (but randomly) just stop working, it would give absolutely no indication of why it was not working.
I have a 4G LTE Network Extender provided free by Verizon. My only issue is calls drop as I leave my property.
I called 911 in January and gave my address before the call dropped as I moved my car from my driveway to the street. The 911 operator called me back once I was back in range.
A few months later Verizon asked me to edit the location data with my address. Hopefully, I won't need to test anytime soon.
I'm surprised the major cell providers are cool with letting randos operate cell towers that back into an unknown untrusted ISP and their customers will automatically switch to when in range. It's unbelievably chill for companies that are usually so concerned about their image and controlling the whole experience end to end.
It seems t-Mobile no longer offers such hardware: https://www.t-mobile.com/support/coverage/4g-lte-cellspot-se...