You are unnecessarily complicating things. Send the one you bought back and buy the correct type of modem/router.
Yeah, I'm just gonna do that. Thanks for your help! Assuming I purchase the right type of ADSL router, will it work out of the box or do I still need to put in the details for PPPoE, etc.?
Just run the setup wizard and enter the login details firstname.lastname@example.org and password BT when prompted.
It will probably work straight out of the box. If not it will have a set up wizard which will guide your through the set up process. You already know the username and password should you need to enter it.
Alright, thank you! I'll send this back and order an ADSL router to come soon.
As was recommended get one that does ADSL/VDSL so if you get the option of fibre then you don't need to change anything. The TP-Link one referred to by @gg30340 will stand you in good stead for the future.