'Forget' the 2.4GHz band's SSID on the phone.
From time to time my iPhone connected WiFi jumps from 5Ghz to the 2.4Ghz.
I have two separate SSID for both and different password for both yet it switches on it's own.
Stop and think about why it's jumping - likely because the 5GHz radio is at the point where 2.4GHz is a better choice.
iOS is pretty good about this kind of stuff...
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