Connecting ariport extreme wielessly to router

[#1]

Hi, I have an Airport Extreme, which I want to connect wirelessly to my PlusNet Thomson modem/router so that I can use the Arport as a Wifi Link for my TV and other devices without built-in WiFi, connecting them to the Airport by cable.  I think I need to use the 'extend a Network' option on the Airport Utility but this just tells me that the PlusNet network (which the Airport can clearly see)  cannot be extended. Can someone please help with sorting this out? Thanks.

[#2]

You probably need to also set up WDS on the TG852n.
http://npr.me.uk/wds.html
In my experience this gives poor result, you may be better looking at using a pair of homeplugs.
http://www.amazon.co.uk/s/ref=nb_sb_ss_i_7_10?url=search-alias%3Daps&field-keywords=homeplugs+adapte...