If you logged in via Steam and you cannot authenticate via mobile app, you won't be able to log into your forum account. Patrick and Doge have created secondary accounts to combat this problem.
It is okay to use two accounts on the forum (only for the purpose of if you lost your phone or phone died) but you cannot hide your identity with this other account. If more than one account is found on and IP and you have not put the alias name people know you by, it will be deleted.
That's all, thank you.