You can’t, directly.
The mesh network (between the Xenon and Argon) is Thread Mesh, which is 6LoWPAN over 802.15.4. The mesh network is an IP network, but it’s isolated, separate from your Wi-Fi network.
The Argon is a gateway between the mesh network and the Particle cloud. It’s not a generic router between the isolated Thread Mesh IPv6 network and your Wi-Fi network, so you can’t make direct TCP or UDP connections between the two.
Also, Thread Mesh doesn’t support TCP, so you couldn’t directly do connection from a Xenon to a TCP-based MQTT server, even if the Argon was an IP router (which it’s not).
In your case, what you’d do it send data from your Xenon to your Argon, either by Mesh.publish and Mesh.subscribe or by direct UDP, and then your Argon would be the MQTT client, uploading requests from all Xenon devices in your mesh to the MQTT server by TCP.