Thank you so Much. For anyone else reading this, i ended up buying a 20gb sim-card from TIGO (79$) just in the arrival hall before immigration. I’m sitting now at El Peradon Surf House with 3G 10/7mb. In Guatemala City i had 4G/LTE coverage and 15/15mb . So everything turned out perfectly.

I don’t know about Belize, but I am in Guatemala and have been successfully using Movistar. For 100 Queztales (about $13 USD) you can get a pre-paid 30 day plan with 1 GB of data, which seems to be plenty if you are mainly using it for email, Waze and a little internet browsing. Movistar has good 4G/LTE coverage in Guatemala City, Antigua and Panajachel. I haven’t really tried it anywhere else. The nice thing is that I believe you can use your Guatemalan chip in Belize (and vice versa) without roaming charges.

Belize has an interesting reputation, it sounds amazing over the internet, and many people talk about Caye Caulker or one of the other Cayes to live in. When you arrive, you will find the country to be incredibly expensive compared to both of its neighbours. My suggestion, head down to the following locations, rent for two weeks each on airbnb to compare, and then decide for yourself where you want to live:
Cancun, Play del Carmen, Tulum, Caye Caulker.

Mexico is far safer (especially riviera maya region), far cheaper, and you have 6 months there for most passports. If I were looking at a long term living prospect, I would look at either living in Playa del Carmen, or Puerto Morelos (which is half way between Cancun and Playa).

Good luck with your decision and keep us posted with what you decide, and also your opinions of the place you do end up picking, as it will help others in future.