Orbi RBR750 Lost Internet (Purple Light)

I fixed it!

Blimey, I spent all day on this, trying a bunch of different things. Turns out the solution was very simple. Very dumb, but hopefully it helps someone else googling this, and this thread comes up:

Basically this was the issue:

https://community.netgear.com/t5/Nighthawk-Wi-Fi-5-AC-Routers/Modem-wont-send-Internet-to-router/m-p/2091527/highlight/true#M190115

I had my laptop connected directly to the modem via an ethernet cable (as described in my OP) alongside the router. Xfinity/Comcast doesn't like this. So I did as described above: disconnected all ethernet cables from the modem, turned the modem off, turned it back on, connected it to the Orbi, and hey presto, it was working (it asked me to reset the orbi admin page password for some reason, too).

What a nightmare. I spent all day on this, inc. driving out to grab someone's old router off craigslist. It was in trying out the second router and encountering the same problem, that it forced me back to consider something was wrong in the setup of the modem, and to google: "Netgear CM1200 Nighthawk not forwarding to router" which led me to the above post. So if you're reading this and this doesn't fix it for you, try googling that and figuring out what the issue is with your modem not forwarding the internet connection, rather than the Orbi not receiving.