Need help with IPv6 settings

After way too many minor issues, I'm doing a reformat-and-reinstall of Catalina on my 2020 MacBook Pro. NOT using Migration Assistant, so it's a factory-fresh machine.

I'm using hard-wired Ethernet to my Cox modem (no Wi-Fi). It seems to be setting up an IPv6 address, which is a format I’ve never seen before.

USB 10/100/1000 LAN is currently active and has the IP address 2600:8800:7d80:1ecd:8b7:b517:f604:10dd

Green light in my Network control panel, but zero connectivity. No Mac apps can send or receive any data, even to "standard" locations like apple.com and fast.com. Under "Advanced", TCP/IP is set to Configure IPv6 automatically. It has found a weird-looking DNS address of 2001:578:3f::30. What settings do I need to change?

(Background: Other machines hard-wired to this modem work correctly, but they get an IPv4 address looking like 192.168.0.240, which is what I'm used to. If I disable the Ethernet port on the new computer, it picks up Wi-Fi and gets an IPv4 address and works fine, but at Wi-Fi speeds, not Ethernet speeds. I don't particularly need IPv6, but I'm trying to stay as close as possible to factory defaults for this install.)

PS -- Cox tech support was completely unhelpful: "the signal is getting from us to your modem. Anything else is up to you and Apple."