Hi Friend,
Here uplink doen't mean that the port connected to the access switch, it means, the internet uplink ( Gateway ) . when it couldn't couldn't hear heart beats from the ISP IAP will switch to the 3G/4G if available but still connectivity with the access switch where other IAPs connected should exist.
bottom line is, IAP with 3g/4g will be the priority but it will form cluster with the subnet where it is connected to the access switch (VLAN). we are giving priority due to the Internet link failover.
hope you got some clarity, please feel free for any further query on this.