Did you try GUI>Settings>Updates>Proxy Settings>Direct connection (no proxy)
The best place to get assistance is right here on the forums.
The only reason I can think of them (third party support) telling you the only forum they recognize is avast(dot)com
is they wanted to rip you off. Our advice, here on the forums, is
free. And we will more than likely assist you
in solving your issue better than them.