Para 15.6 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:jejb1:Tumbleweed/15.6/home:jejb1:Tumbleweed.repo
zypper refresh
zypper install remmina
Para 15.5 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:jejb1:Tumbleweed/15.5/home:jejb1:Tumbleweed.repo
zypper refresh
zypper install remmina
Para 15.4 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:jejb1:Tumbleweed/15.4/home:jejb1:Tumbleweed.repo
zypper refresh
zypper install remmina