Nighthawk M1 + Prepaid Super Slow

Set up the Nighthawk M1 with a prepaid card "Unlimited Mobile Internet with up to 30GB of 5G & 4G LTE Data" - got speeds of 20 mbps. One day later, I'm getting throttled down to 1 mbps at around 5 pm - 9 pm. Only used about 10 GB. Then at 11:00 pm I'm back to 20 mbps.