We'll take a shot at it. This is for a vacation home setup, so I'm trying to get going without having to upgrade the AX. OK, the modem is ISP provided, with a single output. I haven't done ANYTHING with the modem, but am using a first-generation Airport Express with the thing right now. I get a green light, but (just like all the other posts) the actual internet connection ONLY works if I plug the ethernet direct to my Macbook. Otherwise, I get the same "looking for PPPoE" message circulating...and never get through.
I've tried various configurations, including the obvious (ie setting up the AX with PPPoE, entering username and password, etc). No luck. My intention is to replace the AX with my first generation Airport base station (currently used for a fiber connection at home) once I get in the Airport Extreme to replace it. So I'm hoping that this is just an issue with the AX, not the first generation Airport systems....
I'm not physically there right now, so will send the precise configurations later today. I had the AX working with an IP-based authentication at home before, but the PPPoE Username/Password combo used by the present ISP seems to be causing me a headache....