Problem using an APN with 4G not available

[Giovanni Parodi]

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?
I know this may be a simple issue for expert people, but I'm really
confused.
Best regards
Giovanni
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