I've been a teacher.


I was a teacher.

I no longer teach, but I used to teach. Can I used "have been" here?

I have been a teacher


You can use either to refer to something which happened in the past, but normally you'd opt for "I was a teacher" (or "I used to be a teacher").

"I have been..." can, according to the British Council, be used "for something that happened in the past but is important at the time of speaking." It's difficult to think of how this would apply to being a teacher, but here's a contrived example discussing a new restaurant:

Person 1: There's a new restaurant in town, is it good?

Person 2: I have been to that restaurant, it is very good.

Person 2 went to the restaurant in the past, but that fact is relevant to the current conversation. Equally they could have used "I went to..."

Using the "have been" form referring to a profession would, to me, imply:

I have been a teacher (at some point in my life)