The HA (high availability) network feature which allows multiple gateways is not yet implemented and that's what the answer was to similar questions in this forum
Argon to Argon Mesh Setup
Multiple Gateways
2 Argon boards on same mesh network?
However, recently a new "feature" was added which should enable you to add an Argon as pure non-gateway node (without its uplink capability).
However, not with the app but via CLI
https://docs.particle.io/support/particle-devices-faq/mesh-setup-over-usb/
This is the post talking about this option