The difference between Cable and DSL is that some DSL modems already have a built in router that when putting on another one can have some complications in getting it to run well. Some newer Cable modems are doing the same thing.
It's preferred that when a modem has a built in router that you bridge the modem, thus turning off the internal router and having the modem act as a stand alone modem that will hand off a public IP address to the new external router, i.e. 615 and letting it do all the traffic handling. Thus why we need to know what ISP service and modem you have so we can better accurately give you help in getting your router set up correctly.
DSL mostly uses something call PPPoE account information that a DSL provider has for each account. This involved a name and a user password that needs to be entered into the DSL modem or if bridge to a router, into the routers internet connection setup. Once this is done, the router and modem should connect well and then you would connect the PC to the router and should have internet.
Cable modems are easier mostly as they don't rely on account information to get connected to the ISP. They automatically handshake with the ISP service and if you connect a router, hand off the public IP address to the router and your good to go.
If you do need some additional help with the modem, you can call up the ISP and ask them to help you out and sometimes they can help you set up the router too, sometimes they won't though. Always good to ask.