My new iPad can connect to the 5GHz but not to the 2.4GHz despite the MAC address be registered

I get the error message "If your network security is managed by WiFi-address (also known ... modify your network settings." Of course I have my WiFi protected via WiFi-address and I have also registered the new iPad 8th generation (2020) in the device list of the "Timed Access Control" of my router. The first problem is why can I join the 5GHz network, why can I not join the 2.4GHz network and get occasionally aforementioned error message? BTW, both channels are set to automatic. Secondly, the error message is cryptic, as it states as a possible solution ", you may need to turn off Private Wi-Fi Address or modify your network settings.". What is meant by "Private Wi-Fi Address" and where should that be turned off? On my iPad or does Apple refer here to the AirPort router? I can guess what Apple might mean with the 2nd option: "modify your network settings" probably means turning off "Access Control" on the AirPort router. A pity this error message is so hard to understand, as a few words more, e.g. on what device "Private Wi-Fi Address" should be turned off, might make this error message helpful. In this form it does not help me in any way.