MWAN3 with linksys MR8300 and openwrt 23.05: no dhcp address for WANB

Hi.

I'm trying to configure MWAN3 on a linksys MR8300 23.05. In the past I've used
MWAN3 with linksys WRT1200AC and swconfig. Now openwrt moved to DSA and I'm
having problems getting it to work.

I've followed the updated instructions, specific for DSA, https://openwrt.org/docs/guide-user/network/wan/multiwan/mwan3 on how to create the WANB interface by removing a lan port from br-lan.
I've copied that part at the end of this post.

The problem is that WANB does not receive any ip address from the DHCP server running on
the 4G modem to which it's connected (it's the failsafe network, the other
one is fiber and is connected to WAN, and it's working).
So, I'm not sure if I'm missing something in the configuration of WANB and its underlying
device or if the switch on the mr8300 is just not compatile with DSA.
Any suggestions?
Thanks

Routers using Distributed Switch Architecture (DSA)

From 21.02 onwards some targets will use DSA which is different and not compatible with the instructions for swconfig. You can find a converting to DSA guide for additional guidance for switch/VLAN management for router targets using DSA.

  1. Go to Network > Interfaces and select the Devices tab. Click configure on the br-lan device.

  2. Remove a lan port from the switch bridge ports option by selecting the menu and unchecking a switch port such as “lan1”. This port will become it's own WAN port.

  3. Apply these changes to remove the selected LAN port from the LAN bridge.

  4. While still on the Devices page, scroll down and click the “Add device configuration”

  5. For device type select “VLAN (802.1q)”

  6. For base device select the lan port e.g. lan1 which was removed from the LAN bridge earlier.

  7. Assign the desired VLAN ID for this device.

  8. Save the changes and apply.

  9. Go to Network > Interfaces and “Add new interface”

  10. Give the interface a name such as “wanb”

  11. Select whatever protocol is required for this interface DHCP, PPPoE etc. For device select the lan port you removed from br-lan earlier.

  12. Assign the new interface to the wan firewall zone

  13. Apply any remaining changes.