Hello. My name is Michael. I will be helping you with your question today.
When did this occur? Was there anything specific that preceded it?
If you haven't already done a reset of the device, I would suggest that first. If you need specific instructions to do it, let me know that along with the exact model number, and I'll provide them.
It makes sense that it would work if directly connected - it's not discovering it over the network in that case.
Have you tried "add server manually" and then adding it by IP address?
You have to know the ip address of the ASUS RT-AC68W. Then you would use that IP address in the Roku.
Make sure the router has the latest firmware on it. Here's the support page for the router: