PPPoE relay not working as expected

Hello,

I need some advice on a problem I found on both the Merlin firmware and the stock one. I have an RT-AC56U, and my configuration is the following:

The RT-AC56u is configured to use PPPoE as the connection mechanism, with the authentication parameters given by my provider, and it works fine. However, I have to establish a PPPoE session on one of my clients, and I thought that the "PPPoE relay" option was made exactly for this...

PPPoE relay not working as expected