No Schwa letter in glyph collection
I found Capital Ə in Latin Extended-B glyph collection, but there is no small ə (Schwa). Do not be confused by small turned E ǝ it is not the same.
For test purpose try to use this text "Əlaqə".
And also, would be great to add glyph by code, if some letter is unsupported.
You can add any Unicode letter or range of letters to your font with the Custom Glyph Range functionality. There are a few built-in ranges (like you discovered, the basic Latin ranges) but right below that you can specify any other glyphs you need.