I had a problem where my new TV computer would stay on 2.4GHz Wi-Fi, making it unusably slow:
Everyone I found while searching said it was impossible to do, or you had to do it by editing the CRDA database to mark 2.4GHz as unusable, or you can do it but by manually editing some cache file to force the exact channel of your network.
LibreELEC uses connman to manage Wi-Fi networks, and then connman delegates actually connecting to iwd (previously, it used wpa_supplicant).
iwd supports being told to pick between 2.4 and 5GHz networks per its manual.
All we have to do is add
[Rank]
BandModifier2_4GHz=0.01
to the iwd configuration.
We can't do that the usual way as /etc is readonly, but iwd supports changing its configuration directory with the CONFIGURATION_DIRECTORY environment variable.
Add a systemd drop-in with systemctl edit iwd, then add
[Service]
Environment=CONFIGURATION_DIRECTORY=/storage/.config/iwd
at the top between the two comment markers.
Create /storage/.config/iwd/main.conf, with the iwd config as above.
systemctl dameon-reload; systemctl restart iwd, and watch as it moves to the right band.
You can also do the same if it's picking 5GHz erroneously - set BandModifier5GHz instead.