How Do I Set Up LTE Passthrough on a MikroTik Router?

Configuring an LTE router for passthrough mode can be highly beneficial, particularly if you want to let another router or firewall manage the network while using the LTE modem purely for internet access.

Passthrough mode assigns the public IP directly from the LTE connection to the router behind it, allowing the secondary router to handle all networking tasks such as firewall, NAT, and routing. This is particularly useful for setups involving multiple devices or complex networks.

What is Passthrough Mode?

In the context of LTE routers, passthrough mode allows the modem to pass the public IP from the mobile network directly to a downstream device (usually a router or firewall). In this setup, the LTE router essentially becomes a transparent bridge between the LTE connection and your network, offloading network management duties to the device receiving the public IP address.

Step-by-Step Guide to Configuring Passthrough Mode

Step 1: Access the Router’s Configuration Interface

The first step in configuring an LTE router for passthrough is accessing the router’s management interface. This can be done through either Winbox or the WebFig browser-based interface. Winbox is a preferred tool for MikroTik routers due to its ability to access routers using MAC addresses, making it convenient even when IP configuration isn’t properly set.

• Winbox: Connect via Ethernet and launch Winbox. The router should appear in the list under "Neighbors," allowing you to connect by selecting the router and clicking "Connect."
• WebFig: Open a web browser and enter the router's IP address, typically 192.168.88.1. Login with your credentials (admin username and password).

Step 2: Configure the LTE Interface

Before enabling passthrough, the LTE interface must be set up correctly. This includes configuring the Access Point Name (APN), which is required for the LTE modem to connect to your cellular network provider.

1. In the management interface, navigate to Interfaces → LTE.
2. Click on LTE APN, and add the appropriate APN settings for your mobile carrier. These settings are usually available from your service provider or can be found through online searches.

Ensure that your LTE modem connects successfully to the network before proceeding to the next step. You can verify this by checking the interface status to ensure a valid connection with the cellular network.

Step 3: Disable DHCP

Passthrough mode bypasses the router’s typical network management features, so you need to disable the DHCP server on the LTE router. This ensures that the public IP address assigned by the mobile network is handed directly to the secondary router.

1. In the Quick Set or WebFig interface, locate the DHCP server option.
2. Uncheck or disable the DHCP server.

Disabling DHCP ensures that the LTE router doesn't assign a local private IP address to the downstream router, allowing the public IP to be passed through correctly.

Step 4: Enable Passthrough Mode

To enable passthrough mode, you must bind the LTE interface to a specific Ethernet port on the router. The most common setup is binding the LTE connection to Ether1, the primary Ethernet interface.

1. Navigate to Interfaces → LTE → LTE APN.
2. Find the Passthrough setting and select ether1 (or the interface you wish to connect to your secondary router) from the dropdown menu.

Additionally, you can specify a MAC address for passthrough. By default, the LTE router will bind to the first MAC address it detects. If you want to ensure a specific router or firewall always receives the public IP, you can enter its MAC address in the Passthrough MAC field.

Step 5: Configure VLAN for Management (Optional)

In passthrough mode, the LTE router effectively becomes invisible in the network. This is a drawback if you need to maintain access to the LTE router’s management interface for future changes or troubleshooting. One solution is to create a management VLAN.

1. Set up a management VLAN on a separate Ethernet port or within the same interface.
2. Assign the VLAN a static IP address from your local network, ensuring you can reach the LTE router via the VLAN while passthrough is enabled.

Using a VLAN ensures that you retain control of the LTE router for future updates and monitoring, without interrupting the passthrough functionality.

Step 6: Apply Changes and Reboot

After all configurations are completed, apply the changes and reboot the LTE router to activate the new settings.

1. Click Apply to confirm the changes.
2. Reboot the router using the reboot option in the interface.

Once the router restarts, the secondary router connected to Ether1 (or the specified Ethernet port) should receive the public IP address from the LTE modem.

Common Issues and Troubleshooting

• No Public IP on the Secondary Router: If the secondary router doesn't receive a public IP, verify that DHCP is disabled on the LTE router and that passthrough mode is correctly assigned to the right Ethernet port.
• Lack of Management Access: If you lose access to the LTE router after enabling passthrough, check if you have configured the management VLAN properly. Alternatively, you may need to connect directly via MAC address using Winbox.​

Configuring LTE routers in passthrough mode is a great way to simplify your network by offloading routing duties to a more capable device. By following these steps, you can ensure that your LTE router provides internet access while allowing your main router to handle the more complex networking functions like NAT and firewall management. Remember to set up a management VLAN if future access to the LTE router is required