hello,
we just bought hundred of GL-AR300m16.
Bad surprise, new firmware 4.3.7 without the API we use to initialize our software. ( /API/router/...).
We try to dowgrade it with admin page, without success.
I can't help with the downgrade process at this point (maybe @bruce can) but wouldn't it be better to modify your software instead of using an unsupported legacy firmware? I mean, just my thoughts as an IT professional here .....
You could try to install the 3.x firmware via SSH instead of uploading it through the UI.
Upload the firmware images via SCP and execute sysupgrade -v imagename then. (See)
Prior it was needed to use u-boot between v3 and v4 they later made changes so it can work with sysupgrade for upgrading upwards only (what i think is still risky), depending on openwrts version itself things can break, the not keep configuration is not always the solution here, there can be partition changes, therefor there is a reason for the verification process.
That command bypasses it.
But i'm confident it isn't backwards compatible, because if they made it compatible to upgrade for a higher version they must have done some type of scripting to ensure it doesn't brick for example re-partitioning to a new layout.