My AX21 (AX1800) offers a choice of WPA3 Personal + WPA2 AES among the protocols in the drop down menu for wifi encryption, and that option works for the legacy devices on my network that only support WPA2, but there is a catch. When initially setting up the legacy device to connect it to the network, I had to have wifi encryption set to WPA2 AES in order for the legacy device to be able to connect to the router. With encryption set to the combo setting of WPA3 Personal + WPA2 AES the legacy device could see the network and allow password entry but it would then say 'cannot connect to network' as if the password was wrong even though it was correct password. So what I did was set encryption in the AX21 to WPA2 AES and try again and they connected properly. Then after they were on the network and given IP by AX21 router I changed encryption to WPA3 Personal + WPA2 AES option in the encryption drop down menu and saved, then waited until router came back online, then rebooted router. All legacy devices (five total) connected to the wifi network normally after that.