Environment : Any IAP with USB port - IAP-3, IAP-108/109, IAP-115, IAP-155
Instant OS version - 3.0 and above
Network Topology: 
Primary USB uplink 3G/4G, connecting to USB port on the IAP which will act as a root AP. Eth1 of root IAP connecting to the switch. Slave IAPs will be connecting to the same switch using Eth0 port.
Answer :
- Bring up the root IAP with 3G/4G/wireless uplink
- This will be acting as a root IAP
- Create a dhcp scope for the wired-downlink on the root AP in a new vlan
- Create a wired profile and assign Vlan 2 to it
- Assign the wired-profile to eth1 on the root AP
- Connect the downlink switch to the Eth1 of the root AP
- Connect the slave APs Eth0 port to the same switch root AP’s eth1 port is plugged into
- All the APs would form a cluster with root IAP acting as Master