For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:derselbst/openSUSE_Tumbleweed/home:derselbst.repo
zypper refresh
zypper install axel
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:derselbst/openSUSE_Slowroll/home:derselbst.repo
zypper refresh
zypper install axel
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:derselbst/15.5/home:derselbst.repo
zypper refresh
zypper install axel