First thing, it's preferred to set up your networking devices in the following manor:
ISP modem > 615 router > switch < pcs and devices connect to switch.
Is your DSL modem a stand alone modem or does it have a built in router?
Its preferred to bridge the modem so that only one router is handling data traffic. Having 2 routers online can cause issues.
If your going to use the 615 as a main router, then I recommend setting up the router for IP address reservations for all of your connected devices on the router. Ensure Devices are set to auto obtain ip addressing.