OpenWRT on DIR-2660 very slow Wi-Fi

I’m using the latest stable version of OpenWRT on D-Link DIR-2660 as Dumb AP on B.A.T.M.A.N mesh network (connected together through 5GHz mesh), and the wifi performance seems to be very poor, any possible fix?
I factory reset and installed a fresh OpenWRT installation as well, but the WiFi doesn’t seem to go beyond 120MBPS on a 300MBPS connection.
The main AP where internet gateway is on MR90X, and it’s working phenomenally well.

you'll never achieve 300MBPS on a 300MBPS connection, what speeds were you expecting ?

On my MR90X it’s 300MBPS.

What's the speed if you connect to the DIR-2660 by cable?

Connected via Cable.

Via Wi-FI (Almost 3FT distance)

I'm on 5Ghz. 36 Channel, and 80Mhz Width.

You need irq steering at least