While updating the repository using apt update, I'm getting this error and I think the issue is on the mirror
E: Failed to fetch https://mirror.nishi.network/ubuntu-ports/dists/mantic/main/binary-amd64/Packages 404 Not Found [IP: 103.152.178.194 443]
E: Some index files failed to download. They have been ignored, or old ones used instead.
More or less, quite simply the repository isn't there.
The front page suggests that the repo owner split up the common i386 and AMD64 repositories, and the other ports into seperate directory trees
You should be using https://mirror.nishi.network/ubuntu not https://mirror.nishi.network/ubuntu-ports for AMD64 clients
