Yes I can figure out how to change the APN if I have to. I just called T-Mobile and subscribed to the $50 / 100GB plan. At first he offered me a 22GB and $85 per month plan, then I told him that I saw a $50/100GB plan on their website, and he switched me to that. I'm not sure why he offered that one at first and why the price difference is so big.
I had a SIM card already that I bought on Amazon, but he said he was going to send me one since he didn't know if that one would work. I said I wasn't sure if the Nighthawk M1 would work, but he seemed pretty confident that it would, with the correct SIM card.
Maybe I should have kept the Nighthawk M1 I bought on Amazon. The Walmart one has an AT&T icon on it and looks pretty used. Oh well.
Just waiting for the SIM card to get here to see if it works.