North American (US) carriers?

What are people using as 4g LTE carriers in the US? I was hoping my prepaid T-Mobile sim would work, but I can’t figure out the APN. I also tried a FreedomPop SIM as you can get 200mb free a month and couldn’t get the APN settings to work either (this one does work in my phone). Just curious what others are using and if there are any good deals out there.