I know about this issue, so during the test I didn't enable the onboard 2.4GHz, and it's headless without any keyboard/mouse, I tried keeping everything in "minimal" to test, but the CF-953AX is really difficult to find a suitable USB3 extension that won't drop speed, luckily I found one, but still it works terribly bad on my Pi4 with OpenWrt 23.05.2 as AP. I am going to try with a Raspbian 64bit desktop as client mode to see how it goes, will also update EEPROM and go back OpenWrt to test again later in the week.
BTW about the Asus USB-AC51, I just did some more little test, though I know almost no one will be using the ancient RPi 1B+, I still have it so I was trying to see how well it work. So I think while it's published as "OpenWrt supported" but in fact this is strongly "not recommended", not even as a travel router.
First thing, when I plugged Asus USB-AC51 to Pi 1B+, the whole thing rebooted! OMG I guess it's related to power? So after reboot the system came back. I must say this is not issue with my power supply because I had a test with Raspberry Pi 3 using the same one and no reboot at all. Note that when I measure with RPi 4 using USB-C power meter this dongle doesn't even use 200mA while on load, so it's not possible that USB bus power exceeding.
Second, I put it in AP mode, so no routing/NAT, simply bridging, AP comes up, firing up a speedtest. And....? System load jumped to 6.00! And the WiFi speed ranging from 50-70Mbps, during the test there was one time that probably due to extremely high system loading the AP disconnected me for 5-10 secs, IRQ statistics showing USB interrupt storm which might explain the whole thing.
So....if you want to make yourself a little travel router? Probably at least picking the ARMv7 RPi 2B.
On RPi 4B with the same Asus USB-AC51 you can easily get ~200-220Mbps transfer rate, so that means using this dongle with RPi 3B+ as travel router should be kind of a most optimal combination (the USB gigabit NIC on USB 2 sharing with WiFi dongle, so roughly 200Mbps throughput still achievable, with the onboard WiFi being extra 2.4GHz AP as backup for legacy devices)