For some reason, my Manjaro ARM install on the Raspberry Pi 4 is missing my 5GHz WiFi. It can scan some 5GHz networks, but not my home network. This never happened before on my Arch Linux install, and is very weird. I am aware that this is probably due to wireless country code being set up incorrectly.
In Raspberry Pi OS, there is a way to setup country codes for wireless devices through raspi-config. Fortunately in Arch-based systems, this process is more streamlined.
Fix missing 5GHz WiFi by setting up correct country code
On your Manjaro ARM (or whatever Arch-based system), install required packages for managing country code:
sudo pacman -S crda
Now, try to set up the correct country code:
sudo iw reg set <country code>
For United States, the country code is US.
Reenable your wireless card, and all 5GHz networks should now appear.
Set up country code permanently
It is worth noting that the above step only set up correct country code temporarily. To make it persistent across reboots, edit /etc/conf.d/wireless-regdom and uncomment corresponding country code.
For me, it is the line WIRELESS_REGDOM="US".
Or just execute the script below to set up country code as US.
echo 'WIRELESS_REGDOM="US"' >> /etc/conf.d/wireless-regdom