It sounds like you're using the "Lato" Google font that's already been hosted online–is this right? In this case, you'll want to link to the font's hosted URL using the "Link to .css file" option in the web font entry. Here's a tutorial that uses the "Roboto" Google font as an example for how to set this up:
Note that you'll want to make sure that the link you're using has "https" instead of "http" in the URL. Once that's set up, the Lato font should show up regardless of the device or operating system your prototype is being viewed on.
I hope this helps! Let me know if you have any questions about this or if the font is still having trouble displaying.