[AC-RT68U] Which modem should I pair with this router?

I recently bought an AC-RT68U router as an upgrade to my home wireless network which I use to connect to my broadband. Frankly, I hadn't noticed the distinction between the AC-RT68U (which doesn't have a built in modem) and the DSL-RT68U (which does), and didn't realise that I would need a separate modem to connect the AC-RT68U.

I'd like to understand more about my options here. As I see it I could either:
1. Use the old...

[AC-RT68U] Which modem should I pair with this router?