ASUS RT-AX53U: Terrible performance and unable to use AX network

I bought this router only to run as OW dumb wireless AP, OpenSense and Pi-Hole + Unbound takes care of the rest.

I have followed the official post https://openwrt.org/toh/asus/rt-ax53u but gosh, the wireless performance hurts.

I am used to transfer files between my phone and Linux/Windows (shared network folder) and from my PC to my 3D printer and the speeds are on MB/s
Since I started using ASUS RT-AX53U with OW, the speed is down to KB/s.

To make matters worse, when setting up the 5Gz antenna, it won't allow me to use the AX network. Well, you can set the network as AX but no device will connect to it.
Both my phone and laptop are WiFi6 certified.

Am I missing something??

The speed is well below of what I should expect from it.

image

802.11a : up to 54 Mbps
802.11b : up to 11 Mbps
802.11g : up to 54 Mbps
WiFi 4 (802.11n) : up to 300 Mbps
WiFi 5 (802.11ac) : up to 867 Mbps
WiFi 6 (802.11ax) (2.4GHz) : up to 574 Mbps
WiFi 6 (802.11ax) (5GHz) : up to 1201 Mbps

At the moment it seems like flashing back the stock firmware is the best solution.
I would appreciate some feedback tho

Thank you

695mbps seems like the normal for wifi6 on this device.
Where's the issue exactly?