I had a similar issue recently, as I changed my security from WPA2-PSK to WPA3-SAE. A device which was working fine then failed to connect.
You have two options:
1) Change your security to WPA2-PSK and leave the SSIDS (of the 2.4 and 5GHz networks) the same, so that both are broadcast together.
2) Change the SSID of your 2.4GHz network (so that it’s different to the 5GHz one) which will split the networks into two. Then, set the 2.4GHz one to WPA2 and the 5GHz one to WPA3 (use different passwords!).
Advantage to (1) - all your devices will connect to whichever network gives the best throughput
Disadvantage to (1) - WPA2 is not as secure as WPA3, but personally I don’t think it matters much
Advantage to (2) - you control which network your devices connect to and have more security on the 5GHz one
Disadvantage to (2) - devices won’t switch to a better network automatically, unless one is totally out of signal range
I use (2) because I like being in control 😁. However, (1) would be best for most people.