People in modern days often use WiFi as a synonym to Internet Access.
What I understand from your point is: Your current speed on your current WiFi network is slower than it should be. You'd like to buy an AC router to make your own network and bypass the provider's WiFi?
You can do that by disabling DHCP and wireless broadcast on your provider's modem/router. You then take a cable to connect the router you bought to the provider's modem by linking the LAN of the providermodem to the WAN of your own device. You set the ISP modem/router's IP address to 192.168.0.1, and the IP address of your own router to 192.168.1.1. This separated your WAN and LAN subnets and makes it work, essentially. The ISP's Modem/router is NOT allowed to see your internal network, to keep it simple. You only plug devices into your own router, therefor. You should be pretty much set with the default settings otherwise apart from changing SSID's and passwords, though.
This way, the modem your ISP provided essentially just Decodes and Encodes, and works like a modem. It then forwards the decoded and encoded packets to your router, which will send it around in your local network.
@LameMan'sTerms