LE13 or LE12.2 nightly?
If LE13, the only repo's we keep long-term are the release ones. Development repo's are version bumped periodically when either we or Kodi make breaking changes, and a month or so after bumping we normally delete the previous one to save disk space. The development cycle for K22 has been long and we bumped the repo a few times so the repo version for a nightly image more than a year old will be long gone. You have two self-build options: either revert the master branch to a similar date point to the image and build the add-on that you need. Or (as Cubox shows almost zero active installs and we disabled CI for it) self-build a current LE13 image and it will be able to access the current add-on repo (shared with all other 'arm' arch devices we support).
If LE12 or LE12.2 .. as long as the nightly is since the main release it uses the production/release repo (which still exists) and there is some kind of networking issue between the device and our repo that stops you from accessing it - or Kodi started before networking was fully up and the initial connect failed. Start with ensuring the boot delay is active in Kodi settings (or make it longer) and then doing a forced refresh of the repo a couple of minutes after boot.