Most of the time, Google Wifi (Mesh) should work perfectly with Simple mode. And unfortunately, there are cases where the simple model may cause Google Wifi to loose DNS capability. Until we (or Google) fixes this problem, here is a simple workaround to enable DHCP mode on Google Wifi.
This guide is in Beta, and for advanced users only. Please be aware that after this setup, the satellites sometimes may appear as "offline" in Google Wifi app, but actually the satellites are working.
Firewalla ip address: 192.168.86.25
Google Wifi's satellite points: S1(192.168.86.123), S2(192.168.86.160)
Step 1. Take a note for Firewalla's IP and MAC address, e.g., 192.168.86.25, 02:01:B0:40:03:8B
Step 2. Open network settings in Google Wifi mobile app. Change DHCP address pool size to 1, which only contains Firewalla's IP address, e.g., starting IP 192.168.86.25 - Ending IP 192.168.86.25
Step 3. Add DHCP reservation for Firewalla in Google Wifi mobile app. Use Firewalla's MAC address noted in Step 1 to identify Firewalla in Google Wifi mobile app.
Step 4. Open Firewalla App, set Google Wifi's satellite points to "Monitoring Off".
Step 5. Change Firewalla monitoring mode to "DHCP Mode".