Installing Debian On The Turris Omnia Router
By default it runs an OpenWRT derivative called TurrisOS, but Debian armhf userland can run on it, too. For now it though needs a non-vanilla (read non-debian) kernel as well as some additional scripts.
Since this means that there is yet no Debian Installer support for the Turris Omnia either, the only known way to get Debian running on it is the omnia-debian project on GitHub. It's a script to cross-compile the kernel and to generate an image which with which you can flash Debian armhf on the Turris Omnia.
See the omnia-debian wiki for detailed documentation about flashing Debian to the Turris Omnia. You can even run the Turris Omnia with dual-boot Debian and Turris Omnia by creating a ?btrfs subvolume on the eMMC.
If you're not keen on cross-compiling and accept to trust images downloaded unencrypted from the internet, you can find an ready-made image on http://aule.elfove.cz/%7Ebrill/omnia-debian/.