For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:DarkWav:mono-backports/15.6/home:DarkWav:mono-backports.repo
zypper refresh
zypper install mono-complete
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:DarkWav:mono-backports/15.5/home:DarkWav:mono-backports.repo
zypper refresh
zypper install mono-complete
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:DarkWav:mono-backports/15.4/home:DarkWav:mono-backports.repo
zypper refresh
zypper install mono-complete