I originally tried using the built-in modem (after configuring the networking during imaging, including ISP details, username and password) but that didn't work. I honestly don't know if the built-in modem is working or not (the seller wasn't sure either, having been given the Tivo by someone else a few years ago).

After that, I re-imaged the Tivo and started trying SerialPPP after making myself a cable.