GL-SFT1200 (Opal) with ZTE (Vodafone) K5161z USB cellular modem

Technical Support for Routers 4G LTE, 5G NR, USB modems
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I've been trying to set up a Vodafone ZTE K5161z USB cellular modem as failover on a Gl.Inet GL-SFT1200 Opal travel router.

Opal is running latest version of router software, based on OpenWrt 18.06 and kernel version 4.14.90. The USB modem indicates it is up to date.

After initially working through PLEASE help with ZTE K5161z - Hardware Questions and Recommendations - OpenWrt Forum, which helped me with the initial setup and figuring out what I needed (to do) to have the Opal recognise the modem. Specifically identifying the cdc_ether identifier and type.

However, following through to the end did not yield a working setup, with the modem still not working nor controllable through either the Glinet web interface nor through the Luci interface.

Some more searching led me to OpenWrt - Failover Router where these paragraphs helped me finish up:

K5161z setting up

  • Return to the web interface, go to Network > Interfaces > Add new interface:
  • Name: LTE (or anything else)
  • Protocol: I had to select DHCP client as the 4G dongle was unfortunately routing the network already. If you are following along and are lucky enough to find a "dumb" modem, follow this to configure the dongle in QMI or MDM mode.
  • Device: select the interface found earlier, in my case, eth1
  • Click Create interface.
  • Edit the new LTE interface and click Firewall settings> Assign firwall-zone: wan
  • Disable DHCP on the LAN bridge interface: Network >Interfaces ->Edit on LAN interface and click "Ignore this interface" next to DHCP.

Hope this will help any others in future.

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Okay, so that actually didn't quite work as expected in the field. :man_facepalming: I dismissed the little voice that queried disabling DHCP on the LAN side, and after reset found I couldn't connect to the Opal.

Reset, research and retry.

K5161z Not mentioned earlier - I used a Windows machine to access the GUI for the modem to set up APN details for my SIM, and to enable automatic connection and roaming.

Opal / OpenWRT

Came across this handy description of adding a failover 4G modem to an (older version of an) OpenWRT router. Given current OpenWRT version for Opal is only 18.06 I figured this would be good enough.

Apart from using eth1 (instead of usb0) for the modem reference, and adjusting the names, metrics and weights to suit my preferences, I followed the guide to the letter. Ended up with an extra unnamed interface in the Gl.Inet GUI (although in the LuCI interface all seemed good); this interface seemed to operate parallel to the Tethering interface. However, where Tethering refused to work previously, I now had full control of the tethering link.

Bit more rigorous testing than last time seems to have everything working as it should. Field testing for my use case will happen at a later stage; I'll update thread with outcome.

  • Edit for language and wording 20250319
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the K5161z presence did not remain persistent across Opal rebooting. Also struggled to have the USB device remain recognised, with dmesg reporting an "overcurrent" condition when the modem connected. Investigation continues.

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This is the hardware protection of the SFT1200 CPU.

If the USB device current is detected, this protection will be triggered and the USB port will be disconnected.

It is the hardware design, this protection cannot be uninstalled through software.

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Thanks for clearing that up. I remain perplexed why the modem sometimes remain connected and other times not, but understand protection mechanism being in place.

Might have to resort to only plugging in the modem when I need to, or start looking for a Quectel replacement.