A diagram which is more descriptive is located here → Configuring 1+1 Backup by High Availability (HA) . The firmware manual also discusses this → https://download.peplink.com/manual/peplink-balance-and-mediafast-firmware-manual-8.3.0.pdf .
We connect the Peplink routers to their WANs via switches as shown in these diagrams. Another switch is located in the LAN-side.