You need to refer to the DSL-520B product manual and review how to Bridge the modem. These are the same UI web pages for the routers so this document should work for the 526 I believe. I notice they don't have a specific page for the 526, yet. You'll need to follow the instructions on how to Bridge the 526. Once thats done, you'll need to enter in the PPPoE DSL account information into the 615 routers web page under Setup/Internet/Manual. Look in the first section and select the drop down to bring up the PPPoE selection. This will give you boxes to input the PPPoE account information. Ensure ALWAYS ON is selcted too.
Once this is entered in, power off the DSL modem and the router. Power on the DSL modem and let it sync, about a min. Power on the DSL modem. You should have internet now.
Let us know how it goes.