The problem is often that your browser cannot reach the GTT-provided name server (DNS) which is required to translate the website name into the internet (IP) address needed to reach it. The permanent solution is to provide a public name server address to your computer to meet the need.
A commonly used public name server address is that of Google: 8.8.4.4 or 8.8.8.8.
To add this name server address to your computer please see one of these tutorials below. Note - this needs some experience so ask for help if you are unsure. You can cause more trouble if you make mistakes.
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You can diagnose the problem more accurately by pasting the following address into your browser where I use the IP address directly not the name so DNS is not used:
http://173.203.187.14
If you get a webmail login site (this is just a random site to use as a test) then you do have an internet connection but the DNS may not be working so try the above solution. If you get nothing then you have some other problem or GTT does.
Another, temporary, solution is to close down your PC and modem, restart and hope for the best...