The Lua plugin installs a global table rmlui, which provides a way to load supported font files from Lua. The function takes a single string as a parameter, the file name of the font to load.

rmlui.LoadFontFace('../assets/LatoLatin-Regular.ttf')
rmlui.LoadFontFace('../assets/LatoLatin-Bold.ttf')

See the RmlUi Lua API reference for other things to do with this global.