5G/LTE Modem Random Dropouts

Installing and Using OpenWrt
1

Greetings all, relatively fresh OpenWRT User here, to cut a long story short, fixed line internet at my home is diabolical, so I currently have a TP Link Deco X50 5G as my Main Internet Uplink. However, I am on course to replace the TP Link with an x86 AliExpress Machine with a Quectel RM520N 5G/LTE Modem. While this has been an uphill battle, everything is currently fine except ONE painful issue..

After an Undetermined amount of time, which can be between 1-40+ Minutes, all network clients and the OpenWRT machine itself lose internet connection. There are no Logs that occur when this happens (in the OpenWRT Web Panel, both System & Kernel Logs), and I am yet to find any sort of consistency to these drop outs. I can safely rule out temperature being an issue, the cooling setup has the entire machine + modem stone cold, power seems incredibly unlikely as the drop outs still occur when limiting the modem to within an inch of its life, as well as the fact that the Modem still reports completely fine with Socat, and within OpenWRT (when the modem is manually shut down, OpenWRT will show the Interface as down).

I am genuinely at a loss with this issue, and I cannot replace my TP Link until this issue is sorted. A Watchdog Script is really not an ideal solution. With Cellular being the Sole Uplink for my Home, and not as a Backup Connection, Stability is paramount.

Safely Rule out:

  • Cellular Signal (perfectly fine on phone, a 4G Router, and Deco x50)
  • Any Data Limits imposed by Cell Provider
  • Temperature
  • Bad Antennas (Work Perfectly on other routers)

Almost Certainly Rule Out:

  • Power Supply (due to previous tests with manually throttling), though I have no "proper" Equipment to test "properly"
  • Changes made within OpenWRT by myself (lost count of how many reinstalls, general network configuration is unchanged from vanilla install of OpenWRT)
  • Bad Module (As Module is reporting and acting completely as expected)
  • Bad Base Hardware (Previously Operated Faultless in another use case)

Is there some flaw or limitation I am experiencing with OpenWRT here, perhaps the Modem does something that OpenWRT doesnt expect and it takes a dive, or some implementation somewhere does not play nice with this style of Uplink, etc? I know it seems like I am blaming OpenWRT in this post, you wouldnt be entirely wrong to assume that (its more than likely I am to blame i'll admit lol), but at this current time, I have exhausted the extent of my own knowledge, and I have ruled out with relative confidence anything else I can think of, with my very limited recent knowledge of OpenWRT, I can only tunnel vision on it being the cause of the problem, either by itself, by my own user error/naivety (most likely), or both.

Modem running is USB Mode while Connected to an M.2 B-Key Slot inside the Machine
I am using ModemManager( + MM Luci, which also reports zero issues)
Modem: Quectel RM520N-GL
Modem Firmware (Latest): RM520NGLAAR01A08M4G
OpenWRT Ver:23.05.5 Target x86 64-Bit (Generic ext4 EFI Combined) if that makes any difference).

Thank You,
Regards, a Noob

2

If thete are no logs how do you know there is no connection?
What did you have against qmi to settle on networkmanager?

3

Hi There!
All Network Clients lose internet connection, and by SSH'ing in to the OpenWRT Machine and running Pings, opkg etc also fail.

Im using Modem Manager over QMI because QMI would work once, but upon a reboot of the machine, it would not reconnect and the Interface would have to be deleted and set up again, MM however, does reconnect after reboots/on the next boot

Thankyou

4

So there was never any problem with qmi?

https://openwrt.org/docs/guide-user/network/wan/wwan/3gdongle

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I originally ditched QMI as like I mentioned, it would work for the session after first made the Interface, and after a system reboot, it would no longer work, it also suffered the same issue where internet was lost after an undetermined amount of time.

I did some more testing since making this thread, I determined that the loss of internet occurred when the IP Lease of the Modem Expired and was dished a new one by the Cell Provider.

I determined that ModemManager was not able to handle this transition to a new IP, and explained why disconnecting and reconnecting to the Cell Network via at+cops, then resetting the interface in OpenWRT would restore connectivity.

Your response prompted me to re-try QMI, to see if it too would fail to handle the transition to a new IP, I set up the QMI Cellular Interface and left a device on my network running a watchdog script to see if and when connectivity would drop. Unfortunately, it did loose connection, just like ModemManager. Though I stumbled across a thread that mentioned running echo "Y" > /sys/class/net/wwan0/qmi/raw_ip.

I also discovered running killall uqmi on Boot also makes QMI Cellular work on Reboot!

Rebooting the Machine (and running killall uqmi), I currently have 15 hours of Uptime with absolutely zero disconnects.

So, it seems my own Naivety, ModemManager, and some slight issues with QMI are responsible, but atleast now my issue is fixed, and I can get back on track to sunsetting the Deco X50.

Essentially, the fix was: Use QMI, Set raw_ip, and run killall uqmi on System Boot

Thank you for your input, hopefully this will help some other noob somewhere down the line :slight_smile:

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