AC1750 Smart WiFi Router Model # R6300v2: I have broadband thru att. new Netgear router won't work

I have broadband (landline/phone connected) internet thru att. The router that came with my subscription won't work right. Keeps loosing signal. Att won't replace unless I rent from them. Too expensive. They told me to go out and buy a new router. I did, obviously. I connected the router to my phone line, and my computer has a wireless signal. But no internet. It wants me to connect to the old modem via ethernet cable. I tried plugging ethernet into old ATT router/modem and new router. No internet. What do I do? Do I have to buy my own modem too? Please help asap!!!