Add greek-ext (as it is already enabled) to OpenSans*, Roboto*, Caudex

I am developing a webpage using greek polytonic. I have imported from google fonts the code for the **greek extended fonts**. However the polytonic letters were displayed in default _Arial, Sans-Serif_.
I had to download and install different fonts from Google and using Microsoft Word I discovered that only **Ubuntu** supports extended fonts. **Open Sans** and **Roboto** do not support although Google says that they do.
Is anyone has gone through the same issue or am I doing something wrong?