I've noticed that, for me and my wife at least, #Signal isn't always reliable for time sensitive messages. Even after exempting it from the Android power saving and everything it's almost like it goes to sleep sometimes and notifications don't pop until we open it up. The #XMPP app Conversations.im however is very reliable. Running a packet capture on my phone shows that when switching networks #Conversations immediately reconnects, but Signal doesn't, sometimes for several minutes.