Extending a ASUS AC3200 with a AC1200

I have an ASUS RT AC 3200 Router and want to extend it, via Ethernet, with an AC1200 (I already have both bits of kit) because I have a wide area to cover

Could someone help me out with the best way to configure them so that DNS etc doesn't clash. I also want to have the same SSID across both routers.

I'm on BT Infinity Broadband, using a OpenReach Modem

Many thanks