If cell service is available in at least one area of the property, you could have a dedicated sim for receiving SMS 2FA and use a 4G router to forward the SMS to an email, e.g. Teltonika have this functionality [1].
The 4G router also has the benefit of being able to use externally mounted antennas. Which might help in low signal areas.
Not ideal, but might at least be a solution for some people.
[1]: https://wiki.teltonika-networks.com/view/SMS_Forwarding_Conf...
While that is a solution someone could use, it wouldn't work for the subject here:
> she usually doesn't even have service 100 meters down the road.