Then, if it's similar to my 2830, go into the 'WAN' menu, select 'Internet Access' then in the 'Access Mode' dropdown for WAN2 select 'PPPoE'
Then click on the 'Details page' button to open the PPPoE tab. On there you just need to select 'Enable', put in your ISP username/password and click 'OK' Your router should then reboot and connect.
I would like to put a bit more meat on the bones provided here. With the BT Modem still used, connect the supplied ethernet cable to the WAN port on 2850. Enter into the router's configuration to the WAN General Setup page and disable WAN1 and enable WAN2 populating it with physical mode of ethernet and physical type of 100M duplux and an always on connection.
Under WAN2 internet access enter your supplied username and password and choose 1442 MTU.
On the LAN general setup page, I don't tend to use the defaults and instead would use, e.g. 1st ip address of 192.168.70.1; start IP address 126.96.36.199; Pool counts 30; Gateway IP address 192.168.70.1 [that becomes the routers IP address]. Up to you if you wish to adopt this concept but you do have to enable the server for devices to receive an IP address automatically.
By default wireless is turned off, so if you want that feature, enable it. Do an AP discovery and select the wireless channel suggested and populate with your choice of security level.
Some of the advanced settings require you to use WAN2 from the default of WAN1; watch out for this. When configuring the router always use an ethernet connection. When you have got the router initially working to your satisfaction, make a back up of the configuration. When you do need that file you can never find it, so I tend to store it on a USB stick left plugged into the router.Keep a hardcopy of your logon credentials.
If you are brave enough, then upgrade the router to the latest firmware but exercise caution in doing this task because it is all too easy to "brick up" the router. The task is very easy using the routers inbuilt upgrade facility. Know the fudemental difference betweem the RST and ALL file because using the wrong one will reset the router to factory deafaults.