Hats off to pppoeconf. It’s all so simple, post wailing and gnashing of teeth. (You think I speak figuratively? Not so! There’s a reason I’m not employed as a sysadmin.) Put ADSL modem in bridge mode, time expenditure until Eureka: some hours. pppoeconf, time expenditure until pon: 1 minute. Done!
And now that I have returned to the land of broadband, I hope never to have to look at the Whirlpool forums ever again.