Set up secondary router with VPN

I am in the US with AT&T as my internet provider. I have just signed up with Nord VPN. As my current router does not support VPN, I have bought one that does, the Asus AC2900 (RT-AC86U).

I am a complete novice at this type of thing, and need solid step-by-step advice how to set the Asus router with Nord VPN, and to have this router connected to my AT&T router.