That $15 per month price with Mint is just for 3 months. It goes up to $25 per month after that.
I use the AT&T prepaid plan for $25 per month. It says Unlimited data but it is then say “After 16GB, speeds slowed to max 1.5Mbps for the month.” so basically it is 16 GB per month with a monthly rollover of 16 GB allowed. I don’t use much data so I usually have about 32 GB available every month. And of course it is prepaid so you pay $300 up front for 12 months.. works for my needs.
My wife tried Mint for a short time but we live in a rural area in Illinois and she had issues with dead spots in our area so we both use the AT&T prepaid plan.