@mattj @daniel @masoud
XMPP by its very nature is a protocol that effectively uses "notifications", as it pushes XML stanzas over a channel.
(so as the mobile is almost certainly using some form of NAT, as long as some form of pinging below the timeout of the NAT happens, everything is fine).

So for XMPP not having Google notifications should make little difference, I don't see how an XMPP client/server would even use them.

Now Signal, OTOH, I have no idea what it does. 🤷