Can't reach internet from simple modem-router-computer setup

Hi everyone,

I'm struggling with a simple modem + router setup and I hope you'll be able to help me. For context, I work in software development but don't have particular networking skills besides things I can pick up when talking to networking engineers.

The modem is a CBN CD8000. The router is a TP-Link Archer C7 v5 (AC1750), running OpenWrt 21.02.0 r16279-5cc0535800.

First things first: if I plug an Ethernet cable from the modem to my computer, I can access the internet (as defined by "I can ping").

Now, to add the router. I'm starting from a soft factory reset after firstboot && reboot now. I'm adding an Ethernet cable from the modem to the WAN port of the router. The router has one blue WAN port labeled "Internet" and four orange LAN ports labeled "Ethernet", so this seems like the right thing.

I take the computer cable, unplug it from the modem and plug into the router's "Ethernet 1" LAN port.

I can now ping I can't ping anymore (Destination Net Unreachable). Same results if the modem is connected to one of the router's LAN ports.

Any idea why this simple modem -> router -> computer wouldn't work?

Also of note:

  • if I ssh into, I can't ping either
  • if I go back to the previous setup where both the router and my computer are connected to the modem, I can ping but not

I found Troubleshooting Internet Connectivity. Step 1 is okay: the WAN connection of the router is connected to the modem. I checked that the Ethernet cable works.

Step 2 is checking that the LAN and WAN interfaces do not share the same IPv4. LAN has the expected, but WAN doesn't have an IPv4 - it uses DHCP:

I tried giving it a static IP of, to no avail.

At this point it sounds a lot like I'm just missing something obvious. Thanks for reading, and I hope you can help me!

Currently, the WAN does not have an IP address, so it is not surprising that you can't access the internet.

You have a cable modem, so you probably need to reboot it -- possibly several times. Most cable modem systems bind to the MAC address of the downstream device (usually your router), and they will not provide an IP address to another device. Rebooting it will usually allow it to clear that binding so it can "learn" the new MAC.

If you are bridging the modem to the OpenWrt device, you might turn off DHCP, wireless, and the firewall on the modem, and have the OpenWrt device do all that work.

Some ISPs will not allow devices that they don't recognize (i.e. MAC address) to connect to their network.

You may also need to clone the MAC address of the modem to the WAN interface of the OpenWrt device.

In LuCI -

Network > Interfaces > Devices tab

Select the device name associated with the WAN interface.

Click on the Configure button.

Enter the MAC address of the modem -

Click Save, then Save and Apply.

Thanks for the prompt responses!

It seems like the one thing I was missing was to reboot the modem. After doing that, the WAN got an IPv4 address and I was eventually able to connect to the internet.

I'll see if I can add that information to that wiki page.

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