In order to conserve the limited bandwidth available, ISO images are not downloadable from mirror.centos.org

http://mirror.vcu.edu/pub/gnu_linux/centos-altarch/7.7.1908/isos/aarch64/

http://mirror.math.princeton.edu/pub/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.genesisadaptive.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.chpc.utah.edu/pub/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.genesishosting.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.dal.nexril.net/centos-altarch/7.7.1908/isos/aarch64/
http://ftp.osuosl.org/pub/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.lax.genesisadaptive.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirrors.xtom.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.facebook.net/centos-altarch/7.7.1908/isos/aarch64/
http://mirrors.ocf.berkeley.edu/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.keystealth.org/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.atl.genesisadaptive.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.calgah.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.esecuredata.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.cedia.org.ec/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.aktkn.sg/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.usenet.farm/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.worria.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.init7.net/centos-altarch/7.7.1908/isos/aarch64/
http://mirrors.nxthost.com/centos-altarch/7.7.1908/isos/aarch64/
http://centos-altarch.mirror.liteserver.nl/7.7.1908/isos/aarch64/
http://mirror.airenetworks.es/CentOS-AltArch/7.7.1908/isos/aarch64/
http://mirrors.huaweicloud.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.nl.datapacket.com/centos-altarch/7.7.1908/isos/aarch64/
http://centos-altarch.itsbrasil.net/7.7.1908/isos/aarch64/
http://ftp.rz.uni-frankfurt.de/pub/mirrors/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.hoster.kz/centos-altarch/7.7.1908/isos/aarch64/
http://mirror.serverion.com/centos-altarch/7.7.1908/isos/aarch64/
http://mirrors.xtom.nl/centos-altarch/7.7.1908/isos/aarch64/
+ others, see the full list of mirrors: https://www.centos.org/download/altarch-mirrors/

Mirrors verified Fri Feb 21 15:35:40 2020 UTC

You can also download the ISO images using bittorrent, a peer-to-peer file sharing protocol. The .torrent files can be found from CentOS mirrors. Various bittorrent clients are available, including (in no particular order of preference): utorrent, vuze (Azureus), BitTorrent, Deluge, ctorrent, ktorrent, rtorrent and transmission. Packaged copies of various torrent clients for CentOS can be found in the repositories listed in the following wiki article: https://wiki.centos.org/AdditionalResources/Repositories