Hi,
Do you have any other router that you could test the Pi3B+ on?
Are you sure that Pi3B+ doesn't need to be set-up?
Does RUT240 show Pi3B+ as connected device and that it was given a DHCP address in the WebUI?
We need any information that would prove that it's RUT240 fault for Pi3B+ not connecting, so please do answer to all my questions above.
EB.