It should be connecting to https://artifacts.elastic.co/packages/5.x/apt when running update if you are using the 5.x repo. You should double check the sources you have configured.
https://packages.elastic.co/beats/apt is used to serve the 1.x beats releases. In any case I can confirm that both repos are still available (tested from Virginia, US). I tested the instructions for both 5.x and 1.x on an Ubuntu 17.04 machine.