I hope your RUT 240 is running the latest firmware version. If not kindly update to the latest firmware RUT2_R_00.07.02.1 for this solution is based on the latest Firmware.
To set security level for WIFI to WPA3
Go to Network> Wireless>
The Wireless networks will be displayed, to the right directly to the WIFI network you want to change there is a pencil sign, click to open. You will get the Interface Configuration window.
Go to Wireless security, on the Encryption choose the WPA3 type you want and set the required parameters.
For RUT240 we have WPA3-EAP, WPA3-SAE and WPA2-PSK/WPA3-SAE Mixed mode.
For more information on RUT240 Wireless and Wireless Security kindly refer to the following link.