I couldn't use the official repo as it lacks support for rtkmips, but I found https://github.com/choryuidentify/openwrt_rtk/tree/ubuntu-20-04 which in turns is a fork of https://github.com/cgoder/openwrt_rtk to make it compile on Ubuntu 20.04.
That's OpenWrt 14.07. It's pretty old, nevertheless I'd like to keep it updated with security fixes and maybe new features, but I have no idea how to update it. The repository isn't updated in 5 years except for build fixes.