iPhone 6 a1586 Europe model using in USA

I want to visit USA this year and I am thinking about mobile carrier which I can use. I've read many site but still no clear answer. This model should works with gsm and cdma. Of course I am using it with gsm in Europe. But is it mean that I can choose only between gsm in USA (AT&T or t-mobile) or I can get Verizon. Also I read that a1586 supports both cdma and gsm but problem is a sim activation. Verizon needs IMEI number and if phone isn't from USA it won't activate. Unless I find sim which was activated before on us phone. Maybe some can give me advice about that.