Follow the below steps to ensure you've properly configured your LB110 or LM1200 gateway modem to work on our AT&T Data capped plans such as the Blue 300 / 450 data plan. Failure to properly configure your APN in accordance with these instructions will result in service line being cut off for the remainder of the cycle (resets on the 2nd). No refunds will be given for failure to follow these instructions and or use the correct APN connection. DO NOT USE APN setting: “broadband” (This will come up as the AT&T default. SELECT OTHER)
Be advised that while changing the sim card itself is relatively simple, Netgear model devices now require a few extra steps (APN setup) prior to service being restored after a sim card change/ installation, as I'm sure you've already figured out.
Most users report the need to log into the LB1120 User Interface AFTER installing the sim card and rebooting the device, in order to select an APN profile to use as its internet connection source. This pop-up window should appear once you enter the device's sign-in password (located on the label of the device). On that pop-up there is a dropdown menu with available/ APN profiles to use. SELECT “OTHER”. You will then need to manually setup the APN Profile. Keep reading for more details about this below.
This can be done using a phone connected to your wifi (assuming your wifi is connected to the lb1120 via lan (hardwire) connection) with the web address login IP of 192.168.5.1. It can also be done using the Netgear Mobile App. If you are using a computer, just open a web browser and type in the IP address (listed above) into your URL bar (DO NOT USE A SEARCH ENGINE) on your favorite web browser.
Step 1- Log into the LB1120 user dashboard. This can be done using a phone connected to your wifi (assuming your wifi is connected to the lb1120 via lan (hardwire) connection) with the IP: 192.168.5.1. It can also be done using the Netgear Mobile App. If you are using a computer, just open a web browser and type in the IP address (listed above) into your URL bar on your favorite web browser. (see picture below of what you should see if above was done correctly)
(Picture taken After sim card change and reboot)
Step 1 cont. - Use the PASSWORD: “xxxxxxxx" listed on your device's label and then enter that password into the "Sign-in" text field. This is case sensitive. If that password doesn’t work, you’d need to factory reset your modem by pressing the reset button on the back (using a paper clip) for 30 full seconds)
Step 2- Once signed-in: (Click Submit Button)
Step 3 - Next should take you to APN settings page (should like like pic below). This is where you create your APN (connection) profile. This tells your device how to route network traffic up to the carrier network.
Step 4- Fill in the highlighted fields with below info (like picture below shows) AND click "SAVE" once info entered correctly:
Name: TMO Static
APN: b2b.static
Authentication: PAP
Username: LEAVE BLANK
Password: LEAVE BLANK
PDP: IPV4
PDP Roaming: None
Step 5- Select the APN profile you want to use for your connection (click the circle that pops up next to the AT&T profile you've just created, just like the picture shows). 
Step 6- Once you've done this, Click on the "Preferences" option (next to the APN option) and ensure you have the "Always connect, except when roaming" option selected. Otherwise, you'll need to continually log in and hit the connect button (which is absurd). (example picture below)
Step 7- Unplug the LB1120 LTE gateway modem from power. Wait 30 seconds, then plug it back into power. If you are using a wireless router connected via LAN to your LB1120 modem, unplug this from power as well.
Step 8- Plug back in power cable to the Netgear LB1120 Modem and wait 2 minutes for it to boot up and you should see the internet status go from Disconnected to Connected. The Power light on the device should also switch over to green.
Step 9- Plug back in the power cable for your connected wireless router and give it 3-5 minutes to boot up. You should have a working wifi connection once both devices are booted up and have done their "handshake".