Question

when does we use "must be"?​

Answer 1

Answer:

Must be is used to say that what some one is saying is very surprising or hard to believe.

Answer 2

Answer:

Must is usually used to talk about the past, as in “The plane must have landed by now”.

...

Your last sentence is not correct because you:

Used must be to talk about the past (be is present tense)

Used two different be-verbs (be and been) to talk about the smartphone.

Used this September to talk about the past.