MTU size issue in android app in singapore

I am facing an issue for login into my android app from singapore. It is working in other countries. From my client, i got to know it is MTU issue which he faced in previous app too. I am using aws server with ubuntu OS for hosting my web services.I have tried every possibility of resolving this issue.I have changed the MTU size to 9001,1500 and 561.Yet no luck.Thanks in advance.