EERO mesh system connected to fiber gateway

To turn on IP Passthrough, or not to turn on IP Passthrough, that is the question...

If you want to use your Mesh router as a router, and use all its features such as time of day traffic blocking, and handing out the DNS server IP of your choice, and not have the dreaded double-NAT... you would need to set up IP Passthrough on the Gateway for your router.

On the other hand, if you bought the Mesh system only for better Wi-Fi coverage, and your Mesh system allows you to put it in Access Point (sometimes called bridge) mode, then you need not bother with IP Passthrough.

What if I have a MagicJack connected to the Gateway?  I have set the Passthrough mode to DHCPS-fixed using the eero MAC address.  I was thinking of connecting the MJ to the eero second ethernet connection.

I followed these instructions:

1. Ensure that your router's LAN is set up on a different IP subnet than the Gateway
2. Connect the WAN port of your router to a LAN port of the Gateway (probably avoiding port 1)
3. Connect some device with a browser to a LAN port of the Gateway (probably avoiding port 1)
4. use that browser to go to http://192.168.1.254
5. Go to Firewall > IP Passthrough, enter the password from the sticker on the side of the Gateway when prompted
6. Select "Passthrough" for Allocation Mode
7. Set "Fixed" for Passthrough Mode
8. Select your router in the drop down for the MAC address
9. Hit Save
10. Power off your Gateway and Router
11. Power on the Gateway after 30 seconds
12. Power on the Router after 5 minutes
13. Check that the router's WAN address is something other than 192.168.1.x
14. Test everything out
15. Turn off the Wi-Fi on the Gateway (Home Network > Wi-Fi)

The IP address of the eero is 192.168.4.1.  I think I am good.

What if I have a MagicJack connected to the Gateway?

That's not a problem.  The Gateway will continue to do NAT for anything not coming from the IP Passthrough device and will work fine.  Or moving the MagicJack device to any open Ethernet port on your Mesh router is also fine.