I haven't experimented with adding a printer to the WRT-1900AC, yet, but thought of one thing that might make a difference for you.
Which USB port are you using for your printer? I suspect that you're using the USB 3.0 port, which should work normally with your printer, but it may be worthwhile trying to connect it to the USB 2.0 port that is combined with the eSATA connection. There's a potential that your software doesn't detect a newer USB port configuration. It's worth a try, I think.
Good luck!