Hello. I plan to buy new router to my home. My internet is 900 Mbps. My home is 40 square meters. I need good wifi coverage and usb port. What to choose: Archer A9 or Archer C9?

Hi all, so i bought this router, but I should get speeds up to 1900 mb wireless due to it kombining 2,4 ghz and 5g, so why do i have to pick between the 2 networks it shows ? and how do i check if it