RT-AC68U stuck on 144.44Mbps

I use a Killer Wireless-n/a/ac 1535 Wireless Network Adapter on the MSI GT72 notebook. And when i look at its speed its at 144.44Mbps when connected to the RT-AC68U. I've followed some guides about changing professional settings but nothings working.

EDIT: i have now factory reset the router. Its getting 866.7Mbps at 50cm away with 4/5 bars on the wireless bars on windows 10. Any idea?

Thanks for your help