If you are using the mobile application, they are sensitive to the certificate being valid. This means a self signed certificate will not work. Personally, I use startssl.com for free valid SSL.
Regarding the direct IP it should work fine however your proxy has to add a allow cross origin header (or least forward it from the original request) in order to work from the direct IP.
If this doesn’t make sense let me know and I can try to better explain.