Why can't I log into my minecraft PE on external servers?

Okay yes, I've done /register and when I come back to it I do /login but then it says Usage: /login and WILL NOT let me log in. I've created about 5 different accounts just to be able to play on external servers and every time I try to log back into one it will not let me, it just says what I've said earlier. Please help. I'm tired of making a new one everytime I go to play.

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