I presume you mean 15 metres rather than 15 miles? If it's miles, just get another connection.
Typically you'd buy a wireless access point and connect it to the router with a CAT5 cable or PowerLine adaptor, but this won't isolate the connection. You can assign a different SSID to the access point, but DHCP, DNS, etc. will still be handled by the router, so security goes out the window.
In my opinion, your best option is to create a private SSID on your router (you may need to replace your current one) and extend that with an access point if necessary. Personally, I'd prefer to put my money into the router rather than buy a cheaper one and have secondary devices hanging off it.
If you can, check the antennae gain of the router, which...