You don't actually say what you found or more crucially what you tried.
If you are able to do a manual update, then the firewall isn't blocking avast.setup, so it may just be necessary to change the avastUI, Settings, Updates, Proxy Settings to Direct connection (no proxy), only if you don't use a proxy to connect.
####
Aa a last resort:
You only need a good search as there are a number of reports of the Ashampoo Firewall blocking the avast update. No matter what was tried, it failed, only when Ashampoo was uninstalled did the update work. Plus whichever firewall replaced ashampoo firewall it was OK.
This was reported to ashampoo (support) by one of those posting in the forums and they essentially said it won't work with ashampoo. I'm not sure if it is the inability of ashampoo to work with a file (avast.setup) that is only created when needed or not. Nor am I sure if this is now resolved in a later version of ashampoo firewall.