Providing more information, but no progress (sorry)...
I am in Australia. I can get Internet over the NBN (National Broadband Network, (mostly) fibre infrastructure). Also over MWB (Mobile Wireless Broadband, mobile phone infrastructure).
My Dodo NBN, about 4 weeks ago, Avast! stopped being able to update. Also could not use the web installer.
2 weeks ago, I moved house (by about 5 kilometres), now have Belong Broadband NBN. Avast! still won't update, cannot use the web installer.
But then connected to Internet using Telstra MWB, and Avast! updates. I can switch back and forth from Telstra MWB to Belong NBN, repeating the update failure with Belong NBN, and update success with Telstra MWB.
Am experimenting further, hoping to find a >ping or >traceroute that fails for one and succeeds with the other.