Cisco (Linksys) EA3500 Setup

[#1]

I'm trying to set up a Linksys EA 3500 cable router with Plusnet Fibre broadband. I have a really weird problem...
I am connected to Plusnet but with very limited access. I can access my email (Hotmail using Windows Live Mail) and some Internet sites (for example, BBC News, The Times, Plusnet and some sites are fine, but ITV News, Facebook, YouTube, Google and most other sites fail to connect. My VPN won't connect either).
Looking through the settings, everything seems to be fine except IPv6 which is reporting blank information (this may be a red herring as I don't really understand networking that well).
I'm looking for some information on the correct setting for the EA3500 or any ideas how to troubleshoot the problems please?
Kind Regards: Terry

[#2]

MTU problem? The maximum MTU for fibre will be 1492 due to the PPPoE protocol. You could also try setting the MTU on your computer, rather than just changing the router's setting.
Edit: people have had similar problems with other Linksys routers, without any solutions:
EA4500: http://community.plus.net/forum/index.php/topic,111595.0.html
EA6500: http://community.plus.net/forum/index.php/topic,110384.0.html

[#3]

Thanks. I'll give the MTU settings a whorl but judging by the other posts I'm not confident. Looks like it will be going back as PC World as incompatible with Plusnet.