I'm in Japan which has a similarly uncompetitive mobile space to the US, and they've been offering unlimited 4G home internet here for a couple years now.
Their solution is to IMEI lock the plans to these mains-power wifi routers the size of an old AirPort Extreme, and QoS them to have lower priority than their pay-per-GB mobile customers. That way it can't be abused for people to use it to replace the (still-expensive) mobile plans, and the lower network priority means that they're just taking up unused network capacity anyway.
Their solution is to IMEI lock the plans to these mains-power wifi routers the size of an old AirPort Extreme, and QoS them to have lower priority than their pay-per-GB mobile customers. That way it can't be abused for people to use it to replace the (still-expensive) mobile plans, and the lower network priority means that they're just taking up unused network capacity anyway.