If you can't connect to runescape, change your dns to google's.
To do that,
Control Panel > Network and Internet > Network and Sharing Center > Change adapter settings > Right click properties on your current connection > Select Internet Protocol Version 4 (TCP/IPv4) > Change DNS to 8.8.8.8 and 8.8.4.4 OR you can use Open DNS 208.67.222.222 as well as 208.67.220.220
If you don't know how to do this, I put images below regarding it.
Googles DNS can be found here: https://developers.google.com/speed/public-dns/
After setting your DNS, restart pc or open cmd prompt and type "ipconfig /release" then "ipconfig /renew"
OR