If there are other hosts, liked a server or computer etc., other than the ISP gateway existing on the WAN1 segment of Peplink Balance, you will need to check the I have other host(s) on WAN segment box, and enter the IP address(es) of the host(s) so that drop-in mode can work as expected. Please see the following diagrams for illustration.
Please follow the below instructions to add other host(s) IP address(es):
Check the box I have other host(s) on WAN segment in Drop-In Mode Settings.
Enter the other host(s) IP address(es) and click the ↓ to add. These IP address(es) should be under the same subnet of WAN1.
To configure Drop-In Mode, navigate to Network > LAN > Network Settings.