I already have whatsapp installed, which keeps me connected. Plus delta chat is really helpful.
I don't know what xmpp is or how it works.
AFAIK. All major clients for XMPP is open source and gratis.
Two good options for Android is Conversations and Snikket. Both are available on F-Droid and Google Play Store. Conversations on GPS is a donationware version though.
On iOS you have Snikket and Monal.
On the desktop you have Gajim. I think it's available for Linux, Windows and Mac OS.
You also need an account. Conversations offers accounts for free over at https://account.conversations.im/. There's probably a lot more services as well.