I’m trying to setup multicast group remotely after device joins. I have seen certain MAC commands to achieve this from this pdf.
In the next ChirpStack Application Server release, the FUOTA logic will be completely removed. Currently you can’t send these commands yourself since these are “intercepted” by the ChirpStack Application Server.
When this release is out, you can schedule the above commands like any other application payload. One example implementation is: https://github.com/brocaar/chirpstack-fuota-server.