I just received one I purchased from Amazon but there is no English user's manual. I can usually figure out the software side but as to how to install my sim card, I am not sure. It's the newer version using Android, not MIUI.
As with all Xiaomi phones, there is no manual. However, the A1 is a plain-vanilla Android phone. To see settings for the SIM fo through the settings menu and look for network settings and connections.