Cheapest cell plan I've ever had was 5GB LTE per month, unlimited talk/text for $20 a month from Mint Mobile (have to buy a year at a time for that price).
I dropped them after a year because they didn't support iOS WiFi calling or visual voicemail.
iOS is finally a fully supported phone so i'm switching back.
Backend network is T-Mobile.
ELEPHANTS! DOING CRIMES!