Drop-in Mode allows the Peplink Balance to be deployed in a network without making any changes to your existing network devices. This feature simplifies the installation by allowing the Balance to transparently and seamlessly work with your routers and firewalls.
Let’s take a closer look at the overall picture of the process before installing the Balance.
Suppose your existing network looks similar to this diagram.
In this example the default gateway router is connected to the ISP. With this IP address the firewall is performing network address translation for the local area network to access the internet and this is its external facing IP address.
Now you want to add the Balance between the router and the firewall.
Under Drop-in Mode traffic is forwarded between the LAN and the WAN 1 ports of the Balance without performing any IP address translation. In this case the firewall will not notice any changes of the IP address for the hosts on WAN 1 and the same for the other way around. Here the IP address to 210.10.10.3 will be used for the Balance’s LAN and WAN 1.
After that you can add more ISP connections to aggregate and load balance across multiple links.
In our case DSL and cable connections will be the second and third internet connections. This should be how your network looks like after installing the Balance under Drop-in Mode.
There is also and Guide by @Alan here
Happy to Help,
Marcus 
