The software repos for centos 7 and centos 8 have been archived to the vault.
As a result the yum.repos.d for AppStream, BaseOS and upgrades need to be updated.
e.g.
baseurl=https://vault.centos.org/8.0.1905/AppStream/x86_64/os/
baseurl=https://vault.centos.org/8.0.1905/BaseOS/x86_64/os/
baseurl=https://vault.centos.org/8.0.1905/Extras/x86_64/os/
Further yum and dnf file should now work.