Can't open certain web-sites

I have a D-link DI 524 router and a D-link DSL modem. I have 2 web sites
that I can't get to through the router. They are PCpoints.ca and my PC
mastercard site. Now I can go online and do do my banking with President's
Choice but when I click on the button to to go to the mastercard or points
site it says the page is not available. No one seems to know what the problem
is. I have talked to D-link. They said to set the MTU to 1400, my network
provider said to set it to 1446. PC banking gave me a few suggestions to no
avail. None of these solutions worked. I can access these two sites when I
use a computer that has a dial-up 56K modem. I have WEP security enabled on
the router and I am able to use the internet from my notebook as well, with a
wireless card, and my LAN network between the computers works with print
sharing etc. I tried reluctantly disabling the WEP just to see if that would
make a difference but it didn't and I need security. I disabled Windows
firewall temporarily. That didn't help. I don't what else to try. Any ideas
will be greatly appreciated at this point. Thanks.