[Apt-cacher-ng-users] Must add server IP to clients' sources.list

[buggum]

Many report being able to append the server's IP to each line of the
/etc/apt/sources.list file on each client, and then apt-cacher-ng
works. Can anyone suggest troubleshooting steps for the the
alternative /etc/apt/apt.conf.d/02proxy method added to the clients?
That is, that 02proxy method apparently does not work for many but
presumably has worked for some. Are there ways to find out where it is
failing and correct it?

It seems simpler to make one change to the clients than to append the
server IP to each line of the clients' /etc/apt/sources.list file.