What's the "jelly" cable? Is it a CAT-6? If so, you can fit an RJ-45 plug at each end. It should have four pairs of twisted wire: Green, Orange, Blue and Brown, each made of one solid-color and the other corresponding one with a colored stripe.
If you have CAT-3, it'll only have six single strands. Standard telephone cable. It can be used, but you will need to isolate each white wire and ID at both ends... I wouldn't even try to use that. Too much fiddling.
To try and use the second WiFi router at the end of the existing cable, first connect the router to a computer with a short CAT-5 cable, open a browser and type in the address: 192.168.1.1 and you should get the sign-in page for the router... asking for user name and password. Once logged in, you can poke around and find the place to reassign the IP addy.
EDIT: DISABLE the house router before you do this just in case you mistakenly get THAT admin page!