Finally after long struggle, i managed to root it.
special thanks to @Roph @NoPurposeInLfe @goeo @Jiren @Brutef0rce
Tips here,
Initial: Please, put more attention on enumerating. If you are lost, follow owasp cheat sheet and you shall find your way in.
User: once you are in, understand what caused the error, how can you solve the error? what do you need to get in? The forum itself has already given the answer. If it is asking for a certificate, just show him the same certificate, he will let you in. Once you are in, just be who you wanna be.
Root: Like what @NoPurposeInLfe said, download the application and enumerate it locally would be much easier. Do check out the mailbox at home too, there are some message left behind that would help you.
Feel free to find me if you need any help…