We went to Verizon and talked with them extensively earlier this year. We were told that with our plan and our equipment - phones - Jet Pack - we can use our stuff anywhere in Mexico we get a signal. It is included in our plan. But it depends on your plan. We have found over the years that we can use a lot of GB per month doing stuff on the internet so we opted for a serious plan with Verizon. It ain't cheap but it gives us features we want. We have not been to Mexico since we got this plan (may not be able to get there this year) so we really don't know for sure. But when we went to Verizon the assured us this plan would work.
I forget for the moment which Verizon plan we have but we get a lot of GB per month and if we do not use them they carry over for the next month - only the next month - not month after month. They insist this plan allows use of all equipment in both Mexico and Canada - as well as the US of course.