Yes get wechat, then get into Air China mini program on wechat. (I have my interface in Chinese) - but there is a button that says something to the effect of online check in/seat assignment and that's where you can get seats assigned. (I bought my ticket on Air China official iPhone app because the website kept going back to the first page after I entered origin and destination.) Enter the ticket number and then name and then passport number you used against the ticket and you can go through.

For seat assignments, they only do this ONCE - you can't then look up a booked seat assignment, let alone change it.