WWW HTTP Post request only works on Android if i'm comnected to Wifi, not on cellular data

Hi all,
I’m making POST requests to a RESTful api. it works perfectly on the Unity Editor, and when i build to android it works when my phone is connected to Wifi.
However, if I am not on Wifi, and only using cellular data (4G LTE), it doesn’t work at all. it returns a 503 Service Unavailable (even thoughthe server is definitely up and running).

Can anyone help with this? Would be a huge bummer if people could only use my app if connected to Wifi.

Using Unity Basic if that matters.

make sure you aren’t hosting your REST service locally