Debian 10: (buster): http://repos.rcn-ee.com/debian/

deb [arch=armhf] http://repos.rcn-ee.com/debian/ buster main
#deb-src [arch=armhf] http://repos.rcn-ee.com/debian/ buster main

Debian 11: (bullseye): http://repos.rcn-ee.com/debian/

deb [arch=armhf] http://repos.rcn-ee.com/debian/ bullseye main
#deb-src [arch=armhf] http://repos.rcn-ee.com/debian/ bullseye main

Ubuntu 18.04 lts: (bionic): http://repos.rcn-ee.com/ubuntu/

deb [arch=armhf] http://repos.rcn-ee.com/ubuntu/ bionic main
#deb-src [arch=armhf] http://repos.rcn-ee.com/ubuntu/ bionic main

Ubuntu 20.04 lts: (focal): http://repos.rcn-ee.com/ubuntu/

deb [arch=armhf] http://repos.rcn-ee.com/ubuntu/ focal main
#deb-src [arch=armhf] http://repos.rcn-ee.com/ubuntu/ focal main

keyring:

apt-key adv --keyserver keyserver.ubuntu.com --recv-key D284E608A4C46402