Thanks for the response … I think I already figured it out. i now have it working ok.
At this point, usecase was just an experiment to see if i was able to issue a multicast downlink frame to a group of nodes that were Class C nodes, and had joined the Senet network, thru OTAA.
Solution appeared to be set up ABP node on the Senet Portal and use that to issue downlink frames to the other nodes that had been configured with multicast enabled and configured for the devaddr and sessions keys of the previously set up ABP device on the Senet Portal
Not sure if that is the only way, but it seems to work so far