Sorry for the delay in response, as you've mentioned that Lato font is not rendering properly in XD mobile app on your iOS device. I would request you to go to "Preference" in the XD app and under "Manage fonts" and see if you're able to see Lato under active fonts.
Please ensure that the font is active on the device. Feel free to search it under all fonts and download it again from the list
Kindly let us know if this works for you, we'll be happy to take it further if needed.