@patch Out of curiosity, have you now made that setting and does it work? (where “work” means either “does what it says” or “solves your problem” or both)
Just as well I asked about the syntax because that is well um … non-obvious.
For the record, we are only grappling with the opposite situation i.e. same SSID used with each band.
Assuming that you (like me) use different SSIDs on each band, it looks as if you can set a preferred SSID and in my experience it will hold onto that SSID until it actually loses it (rather than jumping around based on highest signal strength) and for this particular problem if you set 2.4 GHz as preferred and you lose 2.4 GHz signal then it is very likely that you have also lost 5 GHz signal (unless you have more than one WAP and your WAPs have different configs) so it would “never” change band.