iPhone12- how to choose Wi-Fi connection?

[starstreak]

So I have a dedicated 802.11ax Wi-Fi name at home and a 802.11ac on another name. How do I get the iPhone to choose the the 802.11ax whenever the connection is better? I could be next to the router but it’ll join the 802.11ac and not the AX Wi-Fi. And I have both set to auto join. The only reason I don’t have the AC set NOT to auto join is there’s a Wi-Fi repeater that supports 802.11ac and not ax. So I still need the phone to auto join to that Wi-Fi.

 

[now i see it]

The same problem exists with regular dual band routers (2.4 & 5Ghz).
The only way to ensure that a device will always connect to a particular router or band is to "Forget" the password for the other one you don't want to connect to. That way it can't auto connect. But that solution doesn't seem to fix your dilemma.