If you're using your cell phone as the hotspot, it's not intended to be a replacement for home internet. The Peace of Mind plan that offers the 10GB of high-speed data includes reduced speeds of up to 512 Kbps once your high-speed data cap has been reached. Also once you reach the 10GB, the hotspot will be disabled as is advertised for that plan.
It's possible that up until now that reduced speed / disabled tethering had not been properly working. It sounds like you are now getting the correct advertised speed after you hit the 10GB limit. 512kbps would be too slow for Netflix to work also. There would be nothing to fix at this point.
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