How can I force the phone to have a fixed IP address on 2GHz and 5GHz WiFi?

I am running an FTP server on my Samsung Galaxy Note 20. On my router, I have given it a fixed IP address based on its MAC address on a 5GHz network. But when the phone connects on 2GHz, it has a different MAC address. My router refuses to give the same IP address to two devices, even if only one is connected. How can I resolve this?