I did this one a friends 2604 DSL router last year. You go into the DSLs modems web page. Select Wizard. There might a a DSL Auto Connect check box. If so, uncheck it. A Advanced button should appear. Select Next. You should see a list of options on the next page. One option should be bridging. There are 2 options I'm seeing on the 2604 model. Not sure what is on yours. I don't remember which one I used. You could try both or what ever yours shows. Select Next on all the following screens. I see there is a spot to put in LAN IP address. I don't remember doing this for Bridging mode.
I would call up your ISP and they can help you bridge the DSL modem to the 855. You will need your DSL log in information that your DSL Modem uses and this will go into the 855 Setup/Internet/Manual section. You'll be changing the DCHP assignment that the router gets from the ISP to PPPoE probably. I would call the ISP and have them help you with the bridging. Shouldn't take too long. Let us know how it goes.
One thing you can try is to put the IP address that the 855 gets from the DSL modem and put it into the DSL modems DMZ, however I would get the DSL modem bridge as this is preferred.