[arsalahmed786]

Can you post your  MyBB forum link here with a test account ?
What had you filled while creating in the forum URL field ? forum.example.com or example.com ?
I guess that might be the problem,
I have a test MyBB forum installed in one of my vpses.. and when I use it through my VPs IP then it works fine (since I used  IP as domain URL while installation), but when I try to use domain which is link to that VPs then also it able to load the forum but I never be able to login through my linked domain.

I guess you might have similar problem ?

i can't submit forum url yet but i'll PM you wait a minute.

actually we'll buy domain name soon and our forum is under constructions

Edit: PM sent