AT&T Unlimited Premium and Hotspot

I recently upgraded to the AT&T Unlimited Premium plan. This comes with unlimited LTE data shared between 3 devices, and this also included 15 GB of hotspot for each device, which is slowed to 128 kb/s once it runs out. We are switching to this as an alternative to the only internet/wifi available in my rural area, which is slow DSL. As my teens are big on online gaming and are both on laptops doing college work, they blow through the 15 gigs in almost a day, leaving them with the unusable speed of 128kb/s. Can we add a mobile hotspot router, such as the Nighthawk LTE mobile router, to our unlimited plan, and will it use the unlimited LTE data or the 15 gigs of mobile hotspot data? Will it be costly to add a device such as this? Much thanks in advance to anyone who could provide a few answers.