404 and 'Repository does not have a release file' on apt-get update for Ubuntu 18.04.5
Whilst not strictly a proxy issue, the problem was indeed network related.
The server could connect by pinging the repository mirrors, but curling the addresses returned a Mikrotik router login page. Upon further inspection, this seemed to be the case for any site. That'll teach me to use ping for testing network connectivity!
Thanks @rm-rf and @Jad for their assistance