Enable Settings -> Untrusted software -> Allow untrusted software.
Download Chum GUI from https://chumrpm.netlify.app/ and use the latest SFOS and aarch64, unless you are on older device that doesn’t support SFOS 5.x or is different architecture.
For OpenRepos go to https://openrepos.net/ and search for “Storeman Installer” and select the one maintained by @olf (should be the first in the list) and download the latest rpm-file.
You can find the downloaded files in Settings -> Transfers and tap them to initiate the installation.