Usually, if you have a connection, and then cannot retrieve info i.e. web pages dont show up, or it is not possible to retrieve emails..etc..etc.. there is a good chance that either your firewall is blocking the services you want, or /etc/resolv.conf is not correctly setup - i.e. there are no nameserver (DNS) entries.
If you are using dhcp - DNS (nameserver entries) in /etc/resolv.conf are not necessary except in rare cases.
If you are using a static IP then you will need nameserver entries in /etc/resolv.conf
You could try starting wvdial from a terminal and posting the output in this thread. That would be a good starting point. Also check that your firewall is correctly set. Ports 80,443,25,110 need to be open to access the net and retrieve and send emails from your email client, at the very least.