T-Mobile One Plan

I've had a TMobile Simply Prepaid plan for about one year and have had no problems using it in Mexico or the US.
The plan with unlimited data, calling, messaging in No. America is $75US/month.
There are other plans with unlimited calling and messaging that start at $45US/month + $5 for Mexico & Canada, depending on data level chosen:
TMobile plans

Verizon also has an unlimited data plan which includes Mexico. But when you read closer you'll see they will cancel you plan if the data being used is in Mexico for two months. I don't know about t Mobil. They've always been Mexico data friendly.
I still use Verizon it just does well. It grabs the telcel signal and I get great 4G LTE and considers any call I make stateside as a local call. I have a Mexican 1 gig data/call plan that was attached to my plan years ago. No end in sight. No clue how that got attached but I'm not big on questioning things that to work in my favor

I have the TMobil plan for North America, purchased in Austin. I have to use it over 50% in the US or roaming charges kick in. I have been told to get a second line that someone will use in the US all of the time and that will make the balance work for me. When I signed up I was very clear that I live here and that 90% of the usage would be in MX. They said no problem. Not true from what I now understand,

Results in more than 50% of your voice and/or data usage being off-net (i.e., connected to another provider’s network) for any 3 billing cycles within any 12 month period;

https://www.t-mobile.com/Templates/Popup.aspx?PAsset=Ftr_Ftr_TermsAndConditions&print=true#EPPUSYD

This might help. I pulled out a bullet from the boilerplate. The link is the entire agreement

The T-mobile plans work very well here until they don't. You will find many discussions about this on the Facebook forums. If your percentage of usage in Mexico becomes greater than your usage in the States, they will cancel your service. I had the plan and it was fine until I lived here full-time for about 3 months. I then got a message that my service would be discontinued because of excessive usage in Mexico.