OK, speaking live from La Paz via Google FI. It works, but I honestly can't even tell if I'm on LTE or what, mostly because I can't figure out this android phone which is new to me. My iPhone clearly shows bars and indicated 3G, 4G, LTE, etc. But I'm guessing I'm on LTE because the speed is reasonable.
Yes, ATT and Verizon both have roaming plans for Mexico and other countries, but in practice I have found them highly problematic in a number of ways. Here's a quick summary after several years of roaming, mostly in Canada, and now in Mexico. Please keep in mind that our goal is to provide data for the boat, so this is a slightly different objective that just casually visiting a country with your phone in hand. There is no doubt that buying a local SIM card can be the best solution, but it can also be the most inconvenient as you will see below.
ATT. Roaming works well and it seems to select the premium towers, e.g. LTE over 3G. But you have to pay $30-$120 for one of a few minuscule data plans. It's fine if you are traveling casually with your phone, but no good for boat data.
We have a couple of ATT iPads and bought the roaming plans for mexico. They pretty much didn't work at all. Useless is the word that comes to mind
Verizon. They used to have a great deal for canada and mexico roaming where for $2 per day you could use your full plan in those countries. And you only get billed for days you use it, so you can just leave the feature active and travel at will. Seems awesome, right. So I set it up for one phone and for a USB modem that we have. Two problems. First is that by all indications you roam on the shitty legacy networks, not the new fast ones. So I would routinely find the modem on 3G when my phone was on LTE with the same carrier. Second, Verizon jacked the prices after maybe a year, now charging $6 per day. So what was $60/ months to extend your full plan to CN or MX is now $180, so getting pricey. And in Mexico, it basely works at all.
ATT has a similar day at a time roaming plan, but it's $10/day, so $300 per month which is pretty rich for what you get.
And there is another problem with Verizon roaming with a USB modem. I plug mine into a PepLink router. Working domestically, it works great, but when operating internationally the modem pops up a warning about roaming charges that has to be accepted, causing the pep link to not connect automatically. The only way to connect is to access the modem via it's web page and accept the popup, and that's required every time is reconnects which is pretty often. It makes it useless for any form of unattended operation. You can configure the modem to automatically connect even when roaming, but it still pops up the message. Of course Peplink and Verizon point the finger at each other.
For Canada, I bought another USB modem and plan from Telus. It worked well, but what a nightmare getting the service set up. You need a Canadian address. The first time, I gave the address of their store and they didn't notice. The second time, I gave a friend's address in BC. That got me service, but now try to pay the bill. To pay with a credit card, your billing address had to be in CN, and it has to match the billing address on the card. Impossible when you live outside of CN. So I tried sending a check made out in CN dollars. It got returned. My bank refused anything but USD. So I estimated the exchange rate and sent a check in USD. The result was a credit on my account for the next two years. Then I reactivated a couple of years later, and hit on a well informed clerk who was able to override their systems, get automatic billing to my US CC, and utilize my old credit. But try doing that a second time....
Oh, and there is the whole myth of moving SIM cards from one device to another. I have never seen it work with a USB modem or myfi hotspot device, and I have tried all permutations of devices and SIMs from Telus, ATT, and Verizon. Each will only work with their own. So just buying a SIM and putting it in the modem isn't happening.
A bunch of people had suggested T-mobile, so I looked into it for our return trip to MX. International roaming is on 3G or 2G, not LTE, and limited to some very slow rate, so unsuitable for boat data. Oh, and hot spot operation is further restricted. It ended up not being any better than buying one of the tiny ATT roaming plans.
Then I stumbled across Google FI. The key for my application is that international roaming isn't on the shitty old networks, and isn't restricted. Plus hot spot operation isn't limited and that's what I need for boat data service. The only wild card appears to be with whom and where they have roaming agreements. So far, at least in La Paz, they are the best of ATT, Verizon, Marina wifi (what a joke), and T-mobile. The down side is that it only works with a select set of phones. I have a spare iPhone 6 that was just sooo-yesterday for my son, and I have been hoping to use it with various SIMs as a hotspot. But no-go with Google FI, so I had to buy a $300 phone. If it works, that's an acceptable one time expense, and if it doesn't work, well, it's the cost of experimentation.
OK, there you have it
