Hallo,
Does TO supports WiFi band steering?
CU Jörg
no_auth_if_seen_on needs one hostapd for both interfaces at the same time. OpenWRT will run one for each interface.
Another option is to deny your 5 GHz capable client on the 2.4 GHz access point.
Third option: choose 2 different names and add the 5 GHz with higher priority.