@Hasan1281
I have no problems with my iPhone 12 Pro Max, but I don't have an AX router either?
I'd set to the SSID it connects to to AX only and see if that helps?
On my routers, a tri-band, I set the second 5Ghz band to AC only and that is what I connect the AC devices to, including the iPhone.
That said, when I got the iPhone 12 I had problems calling or receiving calls on the LAN connection. Internet worked fine. My Cell provider, T-Mobile, had to reset the configuration and the SIM (was taken out of the old phone and put into the 12) and that didn't work. They then had me put in the SIM that came with the phone and that solved my problems with calling.
I recall searching the web and others had calling problems with the iPhone 12 as well. Some were for iPhone's in general though.