I found that even the tar can be used to upload firmware. So tried to upgrade to testing but the installation of 6in4 did not work.
Finally installed this firmware GL.iNet download center and was able to install 6in4 after opkg update via SSH
I would suggest suggest to add to firmware index a few words to explain what is the use of the particular firmware. For example “to install packages via opkg” or “openvpn ready” or whatever. I spent a couple days trying to find out that and was lucky, but many people will give up.
Best
Best