Here in Munich people were seeing and reporting that DM can only be sent in firmwear 2.5 if the other node has at least one private channel that the sending node has too. I posted to ask whether this was needed and got no answers. I fear that this effect needs testing by having both nodes only have longfast channel. Then only one have a private channel. Then only one have private channel as primary. Then recipient only a private channel. Also primary and not primary. Then both have private channel as secondary. Then one as primary. Then both as primary… I gave up testing. I only have so many hours in the day. On my Android app (old version) I cannot SET which channel to use to send a DM. But for example a DM from and store and forward node (like the T-beam is capable of), that received the message FS, will come back to the node that sent the message FS, on the private channel the two have in common. One test with S&F showed that any only private channel in common has merely to exist - not be used - for S&F replies to make it through. Very head-scratching. Am very most mightily perplexed but that seems to be normal - else this Meshtastic becomes a full-time job.
Related links:
say make sure the two nodes explicitly have set the same slot
had no replies
https://www.reddit.com/r/meshtastic/s/sIBwSOUe3b
says not needed
https://www.reddit.com/r/meshtastic/s/uJjF2vmIc4
As an amusing, perhaps, related issue: I was testing S&F and it was not working. In fact it was: just the DM came back on a different (private) channel to the one used to send. And that worked only when primary was the private channel (if using 2.4 and 2.5 mix).