How to configure Nighthawk M1 as 4G Modem only

Netgear Nighthawk M1 to Apple Airport Extreme

I searched a lot and didn't really find the answers I needed all in one place so here is my setup in case it can help someone.

Previous setup: Cox cable modem with outgoing ethernet cable into the WAN port of an Apple Airport Extreme (set to DHCP and NAT mode, using 2 Cox nameservers for DNS)

Goal: To simply put the AT&T Nighthawk M1 in place of the Cox cable modem. WiFi on the M1 would ideally be OFF so it's just supplying internet through the ethernet cable to the WAN port of my Apple Airport Extreme, same as it was getting from the Cox cable modem. Since the M1 only supports 20 WiFi device connections, my preference was to just use the M1 as a modem and have my Airport, which allows 50 connections, handle WiFi and DHCP.

First Consideration: DNS. Before I ever disconnected the Cox cable modem, I signed up for a free personal account with OpenDNS so I wouldn't have to worry about using Cox or AT&T's nameservers for the remainder of my setup. This was something I had always wanted to do anyway and had just never gotten around to it so it seemed like the perfect time to do it.  On my MacBook Pro, I entered the 2 OpenDNS nameservers into my Airport settings using Airport Utility (located in Applications > Utilities):

M1 Settings: 

1) Since I knew my goal was to turn off the M1's WiFi, I connected the USB cable's Type A end to my MacBook Pro and the Type C end to the M1. Type 192.168.1.1 into your browser to connect to the M1 and login. 

2) Click Settings > Setup > Mobile Router Setup

3) Under IP Passthrough, select "ON Disables Wi-Fi on the device". I recommend downloading the M1's manual (MR1100_UM_EN.pdf) and reading about IP Passthrough.

4) Under DHCP, uncheck "Use mobile router as DHCP server"

5) Some M1 restarts may be necessary when changing the settings above. After all restarts are finished, login into the M1 again and go to: Settings > AT&T Wi-Fi Status > Network and record the IPV4 settings, your IP address will differ from mine.

6) Log out of the M1 from your browser and disconnect the USB cable from your computer

7) Connect an ethernet cable from the ethernet port of the M1 to the WAN port of your Airport

Airport Settings:

1) On my MacBook Pro, I launched Airport Utility and kept Router Mode as "DHCP and NAT", the same as in my previous Cox cable modem setup.

2) Note the DHCP Range. The M1 uses 192.168.x.x for its local IP addresses so you DON'T want to use that range on your Airport. Click the Network Options button:

3) I recommend using 10.0 and 172.16 as above so neither range conflicts with the 192.168 range the M1 is using.

4) My Airport didn't "recognize" the IP address of the M1 the way it did with the cable modem so I had to change Connect Using to Static and enter in the IP address and mask from the M1 (see above: M1 Settings > Step 5) in here. Note that the Router Address should be the same as the IPv4 Address, but instead ends with 1.

5) The weakness in this is that if the M1 restarts it will get assigned a new IP address by AT&T. Then I'd have to log into the M1, get the new IP address, and re-enter it in Airport Utility as both the IPv4 Address and the Router Address.

6) At this point it was all finally working as desired! M1 had WiFi turned OFF and was acting as a modem, supplying internet through the ethernet cable to the WAN port of the Airport. The Airport was the DHCP server and had WiFi and guest WiFi both working, giving me 50 device connections.

7) The Airport Utility gave me a yellow warning light by the Status, showing "Double NAT". I just clicked on the Status and selected "Ignore". Again, no expert here, but what I've read told me others have lived just fine with this warning ignored for years. Since the M1 uses local addresses in the 192.168 range and I set my Airport to assign 10.0 on the main WiFi and 172.16 on the guest WiFi, there won't ever be an address conflict. If anyone has networking experience and could comment more on this, I'd like to learn your thoughts.