Well you should have no trouble getting a GPS signal in (I will assume) South America.
However, I doubt the researcher will get anything resembling a data signal on a phone or tablet, depending on how far from civilization.
When you are talking off set, how much are you talking about? I would think that for frog observations that centimeter accuracy is not needed.
You should find out specifically what they need to collect.
For the most part, if they have a device, they know their area of interest (geographically) you could set up off line maps and air photos and they could use the cursor on their phone to ball park the location pretty good. Well canopy cover would hinder that.
Frontier Precision is a company local to me that provides excellent GPS solutions for a wide variety of needs.
https://frontierprecision.com/
If the researcher does need to be super accurate you are probably going to go above their budget.