Probably I'm not the best to answer this, but anyway.
The best I could get with this same hardware was
8.1 MB/s download
6.5 MB/s upload
connected to my NAS. And this obviously quite far from the standard.
I can only recommend you to use WPA2 Security with AES and if its possible in that router, set the Channel width to Auto, instead on 20 MHz only.
Good luck.