M5stack paper not showing German characters "äöü"

[ScheduleDisplay]

Hello,
I Got the GFX font header file (by converting the Roboto.ttf file). Loaded the font file using canvas.setFreeFont(&Roboto_Medium50pt7b) and then display the text using canvas.printf(textString.c_str());.
It is working fine and display the text properly but when I am giving some special characters äöü then they are not appearing on M5stack paper.
Does anyone know what could be the problem? am I missing something?

Thanks