for me the following worked in our company network with proxy:

cd /etc/apt

mv ./apt.conf ./apt.conf.bak

this is to force the system to use the new file we create now.

cd ./etc/apt/apt.conf.d

create new file "95proxies" and add there the content of the apt.conf file
Example:

Acquire::http::Proxy “http://proxy_url:proxy_port/”;

Acquire::ftp::Proxy “http://proxy_url:proxy_port/”;

now the system use the 95proxies file

Try

sudo apt-get update

this should work now