We are using OpenWRT and Coovachilli so it should be supported as you noted.
We added this similar to Teltonika however facing a few issues when logging in after being redirected i.e error cannot reach gateway error
It seems perhaps the uamserver, uamhomepage, and few other settings need to be confirmed. Furthermore need to identify if it needs direct access to HSNM instead of via the public IP.
You you perhaps have any sample config file for coovachilli from openwrt?