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. We use 188.8.131.52
After the change, please tap on Apply.
Install Firewalla using https://firewalla.com/install
After scanning, you can tap on "Manual Setup"
After, Please go to Synology admin page and disable DHCP Server
Some device may take a while to migrate to the new network space. You can rejoin your iPhone/Android to Wifi, the IP address will change to 192.168.218.x