drivers: modem_cellular: Automatic APN detection support

[petrosyan-van]

This PR adds run‑time APN selection to the cellular driver and the logic that programs the chosen APN into the modem before the normal dial sequence starts.

Highlights

• New Kconfig options

  • CONFIG_MODEM_CELLULAR_AUTO_APN – enables the feature
  • CONFIG_MODEM_CELLULAR_AUTO_APN_PROFILES – string table mapping MCC/MNC codes to APNs (same as used in drivers/modem/ublox‑sara‑r4.c)

• APN detection flow

  • After IMSI is read, the driver performs a six‑digit, then a five‑digit MCC/MNC lookup in the profiles table.
  • On a hit, the detected APN overwrites the static one; on a miss, the static APN is preserved.

• Dynamic “set APN” chat script

  • Introduces a new driver state RUN_APN_SCRIPT inserted between open DLCI2 and run dial script.
  • modem_cellular_build_apn_script() builds the chat script at run time using buffers that already exist in struct modem_cellular_data.
  • Every modem executes AT+CGDCONT=1,"IP","". Sierra Wireless HL7800 additionally receives AT+KCNXCFG=1,"GPRS","",,,"IPV4" (added via DT compat guard).
  • On script success we move to run dial script
  • On failure we continue repeating?

• Testing

  • Evaluated on a Quectel BG95 with Swisscom and Emnify SIMs; both boot to PPP with the correct APN.

I used this commit: fdd89ef from @hwilmers as inspiration.

My solution is not as elegant at all, as I couldn't find a really good way to handle the dynamic script build. i saw several discussions previously regarding this topic and some attempts to implement this feature, but for some reason the idea was dropped, perhaps due to the solution not being scalable. I found this discussion and PR here:

#67626
#67776

Nevertheless, I think this feature is important as it removes the need for per‑carrier firmware variants, which is a major practical gain. Feedback on improving scalability or restructuring the builder is highly appreciated :)

Note: Would be great to test the solution on other modems, especially on HL7800, which has 2 steps for APN script. Any help there would be great!

[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[jeffwelder-ellenbytech]

This is really interesting!

A while ago I made some brute force edits to the modem_cellular state machine on a fork to switch between a primary and secondary APN with the cellular.h api to switch between a primary and secondary chat script.
(mpenate-ellenbytech@cd216ae)

Couple other bg95 goodies I never got time to implement are here:
https://github.com/mpenate-ellenbytech/zephyr/commits/v3.7.99-eti?author=jeffwelder-ellenbytech

[petrosyan-van]

This is really interesting!

A while ago I made some brute force edits to the modem_cellular state machine on a fork to switch between a primary and secondary APN with the cellular.h api to switch between a primary and secondary chat script. (mpenate-ellenbytech@cd216ae)

Couple other bg95 goodies I never got time to implement are here: https://github.com/mpenate-ellenbytech/zephyr/commits/v3.7.99-eti?author=jeffwelder-ellenbytech

Thanks for the feedback! You have some interesting stuff there :)
Would be great if you can get some of your improvements merged at some point, especially the Network Tower info. This is something that I actually need in my project :D

I've been also focused on BG95 improvements recently. Funny enough some things we did are matching, like those:
#93068
#93072

[bjarki-andreasen]

Instead of making this a hardcoded part of a single cellular driver, I would highly recommend extending the cellular.h API with an entry like

int cellular_set_apn(const struct device *dev, const char *apn);

and allowing the application to store the list of APNs it knows of, either statically or dynamically (up to the application).
To know which APN to set, the application can already get IMSI itself using

zephyr/include/zephyr/drivers/cellular.h

Lines 215 to 217 in 6413185

	static inline int cellular_get_modem_info(const struct device *dev,
	const enum cellular_modem_info_type type, char *info,
	size_t size)

and do whatever lookup it needs :)