Problem using an APN with 4G not available

[Aleksander Morgado]

Hey,

> Good morning, I'm a user of libqmi and qmicli tool and I need some help.
> I'm using a SIM of an Italian provider (TIM), configured on a standard private APN (the SIM and APN are of a customer of my company, he pays for a custom APN to the TIM company).
>
> When using the standard public APN (ibox.tim.it), with a modem that supports LTE connectivity (Quectel EC25), I can go online and get a connection to the network.
> If I configure the custom private APN (that uses the same network infrastructure), I'm able to connect to the (private) network and to go online, pinging the private customer server, only if a set the modem as 3G using the command line:
>
> qmicli -d /dev/cdc-wdm0 --nas-set-system-selection-preference="umts|gsm"
>
>
> If I configure the SIM as:
>
> qmicli -d /dev/cdc-wdm0 --nas-set-system-selection-preference="lte|umts|gsm"
>
>
> I receive the error:
>
> <<<<<<   type       = "Call End Reason" (0x10)
> <<<<<<   length     = 2
> <<<<<<   value      = F5:03
> <<<<<<   translated = gsm-wcdma-unknown-apn
> <<<<<< TLV:
> <<<<<<   type       = "Verbose Call End Reason" (0x11)
> <<<<<<   length     = 4
> <<<<<<   value      = 06:00:1B:00
> <<<<<<   translated = [ type = '3gpp' reason = '27' ]
>
>
> Here is my problem: if I use the SIM in a smartphone, the smartphone fails connection in lte mode but after a few seconds (about 30) goes to UMTS mode and gets its address.
> My problem is that, even if I select to support umts,LTE,GSM, when the modem tries connecting, it tells me unknown-apn, and does not switch to UMTS connectivity option.
> If I use 3G mode, everything works fine.
> So here is my question: how should I manage this kind of problem?
> How can I force qmicli to manage switching at connection time different mode search of APN?

If you know in advance that you're going to hit the situation, I
assume you could run the qmicli commands to prefer (or even limit) 3G
depending on the specific APN to use? Or, otherwise, just assume that
if you ever get the "gsm-wcdma-unknown-apn" call end reason you should
think of falling back manually to 3G and retrying.

> I know this may be a simple issue for expert people, but I'm really confused.

I am no expert on the inner LTE stack details, so I cannot comment
much more I'm afraid. This could also be a lack of planning in the
operator side, for what it's worth; i.e. why does the APN get reported
as unknown when using the LTE infrastructure? Is that a known
limitation of that APN?

-- 
Aleksander
https://aleksander.es