I have recently obtained Google Fiber Business with multiple Static IPs, to replace my soon-to-be former Spectrum Internet.

The IPs are structured like these dummy values:

IPv4
IP Address: 140.121.224.110
Static IP LAN Subnet: 140.120.224.0/29

So, when I plug my ER605 into the Fiber Jack it connects and gets the IP Address (140.121.224.110).  This works with no problems, and all my hardware has access to the internet.  My problem was getting the static (subnet) IPs to work.

I’ve found if I set my WAN IP to Static and insert the same values as were set when it was DHCP, I can use one-to-one NAT with no problems and the appropriate IP addresses (140.120.224.1 and 140.120.224.3) are used by the respective LAN clients, when performing a "what's my IP address" search.

BUT, this is not the way Google Fiber says I must have it set up.  My WAN must be set to DHCP.  And as I found out after leaving it this way, it stopped working after several days.

My ideal would be to use one-to-one NAT, but since I’m forced to use Dynamic IP for WAN, this is not possible using the ER605.

I have searched for solutions to this problem, and am seriously confused.  I have copied the following off the Google Website.

If you have a multi-layer router or firewall and router configuration, you must set up a static route to the first device. Follow these guidelines:
Make sure the router connected to the Fiber Jack is set to DHCP and that one of its ports is set with one of the static IPs as the gateway address.
Make sure the remaining routers have static routes pointing to the gateway.

This is the page this comes from (and contains a picture of a multiple Static IP setup):  https://support.google.com/fiber/answer/6136162?hl=en

I’m aware that the ER605 has an option to set a “Static Route” under Transmission, Routing, but am unsure how to use/interpret the instructions above.

What steps would I need to take to implement what Google says I need to do?