How I send SMS in West on att?

I have unlock phone I had before I move America. Phone I have run COS (China Operating System) and record to 4k video and have 23 mp camera on back. I pay 107493NTD get it because it exclusive limited edition. There 1200 company ever make. Smallest photo I take 5mb, smallest video 50-75 mb. Smallest SMS photo 5mb and video 25-40 mb. But now I with att person tell me I not able send video or photo that size in USA. So how I send photo and video? Is way extend size limit or jump over limit?