If the question is still open. TLDR: yes.
I have connected my Asus router via WAN as my main router (Fritzbox). This should also be possible with any other router.
Otherwise you can use PPPoE :)

Not sure if the ASUS router can do this as I only use it as an access point. I have set up a VPN directly with the Swisscom router. This is very simple. Here are the instructions: https://www.swisscom.ch/de...

Maybe this will help.