Hi @MaxS0niX,
If you are interested (and have console access) I can send you the openwrt 21.02.RC I have created. It actually works well.
At the moment I am trying to understand the structure of the update file.
For the Sercomm S3 there is a header with a string with the 1st line that represent probably the model number followed by a 16 byte sha256 that is just the calculation of the firmware in gzip format.
The Turbo+ looks similar exept that sha256sum (or what I think it is) does not match anything I tried. I am still trying to find out what it represent. This way we could have a firmware that can be updated through the web interface.
These are the files to be written to mtd4 and mtd6. !!!!!PLEASE USE THEM ONLY IF YOU HAVE ACCESS TO THE CONSOLE!!!! as since the header is done in a way to always work you won't get the uboot protection that swtiches to the second boot kernel anymore.