There is a possibility that your IPV6 is enabled and causing some issues. Can you try disabling IPV6 on your ASUS and try again?
Under Settings>Roon Arc, is the port manually configured to 55000? Go ahead and set it if not. It’s possible that it’s blocked by your ISP. You can verify this by going to https://canyouseeme.org/ and providing 55000 in the Port to check field.
Would you mind sharing your ISP and your modem make/model so we can check some other possibilities?
It sounds like your ISP is blocking the port then. I looked and it seems the Experia box has a UI that should allow you to open the port. I found this website and the steps look good but I don’t have your modem and can’t verify.
Alternatively, let your ISP know that you are trying to port forward. They may have something they can do on their side. At the very least, give you exact steps to open the port. If the canyouseeme site cannot find you on the port, Roon ARC won’t be able to either.