If you want to set up a VPN server your Swisscom box may be able to do this on its own already, as per the other reply here. This Asus router is also able to do this, but then it's best to set your Swisscom box to bridge mode (if this is possible) so your Asus VPN server is reachable from the outside. If you want the Asus router to connect to a VPN server elsewhere (VPN client), then the answer is yes, it is possible, even if it is behind a Swisscom router which is not in bridge mode. I use it for this purpose myself.

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.