Are you sure you don't have an R8000 rather than an R8000P? As far as I can tell, the only alternative firmware for the R8000P is OpenWRT, and the documentation

Are you sure you don't have an R8000 rather than an R8000P? As far as I can tell, the only alternative firmware for the R8000P is OpenWRT, and the documentation claims only 802.11b/g rates are supported. DD-WRT and Tomato, which would have better driver support, only target the R8000. At least I had fun connecting to the console UART in order to flash the latest firmware on this R8000P. The one I bought was stuck on an older version with a bug that prevented it from accepting firmware ever again.