Can you try visiting
http://192.168.1.254/index.cgi?active_page=9121 in a browser on your home network please? It may be that this doesn't work, but if it does, can you upload a screenshot (press Alt+Print Screen/Prt Sc if you're using a Windows PC, then open Paintbrush/GIMP/Photoshop/etc and paste in the image, save it as a JPG or PNG and upload it to a reply in this thread).
I've actually got the same network hardware as you I think, but with PlusNet's case and firmware, there seems to be more configuration compared to BT's firmware.
If that address has other VPN related pages linked to it, please repeat the screenshot process for those as well. We'll have a look at what is available, and see if IPVanish's existing documentation can be applied to your case. If not, we'll figure out alternatives.