Step 1: Change DNS on Router
Login to Synology Router and go to Network Center. In Local Network, go to General. Here you will need to change the default DNS from the Gateway address to something else, e.g. 126.96.36.199 or 188.8.131.52
After the change, please tap on Apply.
Step 2: Setup Firewalla Box with DHCP Mode
Follow the Installation Guide to install Firewalla Box.
After scanning the QR Code, tap on "Manual Setup" and setup Firewalla with DHCP Mode.
Step 3: Disable DHCP Server on Router
Disable DHCP Server on Synology Router. Login to Synology Router -> Local Network -> DHCP Server -> Disabled
Some device may take a while to migrate to the new network space.
You can rejoin your devices to the network to get IP addresses from Firewalla's overlay network, the IP address will change to 192.168.218.x