Update: I switched my parents back to using their old Huawei 3G modem which is locked to a service provider (Vodafone) and had to pay $150 for 75GB of data, because their account balance had been...

Thanks for the suggestions. I tried changing to the 3G setting but the modem doesn't pick up the ISP network. I haven't tried setting a data limit, because I don't think this would fix the problem...

I bought a 4G LTE for my elderly parents to replace an older 3G mobile Wi-Fi dongle that they use for basic Internet access at home. Their Internet usage is minimal and they would typically get...