3G link comes UP but no IP, then goes down and again. BB 14.07, TP-LINK

Hi there,
I try to configure 3G backup link on my TP-LINK MR-3220 with (ok, it is ancient, but fits 4/32M device) Barrier Breaker 14.07 (OOB or self-compiled). I use Huewei E3131 USB modem, card (working in my Fedora 24 laptop) from Play Polish operator.
Config (network is here:

config interface 'wwan_Aero2'
option proto '3g'
option device '/dev/ttyUSB0'
option apn 'darmowy'
option delegate '0'
option peerdns '0'
option defaultroute '1'
option keepalive '10 5'
option auto '0'
option service 'umts_only'
option metric '30'

Symptom: - 3G link configured via LuCI, as 3G/EV-DO, kernel modules present, usb_modeswitch executes (there are /dev/ttyUSB* nodes), modem responds in comgt, link comes UP (can see "3g_wwan_Aero2 connected to /dev/ttyUSB0" in log) when activated trough LuCI, can see it in ifconfig, but no IP address / route / DNS. It stays in this state for 10-60secs, then I see "modem hangup" in log, and it goes on again after few seconds, and this indefinitively.
Same modem / card works OK in Fedora 24 laptop, activatink link trough NetworkManager GUI.

What am I missing? What should I check?

This version is old and unsupported. It will be difficult to find someone to help you with your problem, as no one will be able to replicate. Better consider buying a newer router with specs adequate for the currently stable OpenWrt version.

I KNOW it is"old & unsupported", but buying new router just for 3G connection is not an option for me. I already managed to have 3220 to be backup link (using "tethering" on Android phone and MWAN3). Now I want to have second backup link, for situation I am out the house (so no phone reachable trough WiFi), for example for OpenHAB.

My old TP-LINKs work OK for the tasks I need (Internet radio, ser2net, AP + WDS, STA + WDS, STA, 1043ND even for USB sticks). I have managed to compile a BB custom version, which still has 200+kB flash free (on 3220). Now the ONLY problem is with 3G, I don't know enough PPPd, to know what can cause behavior I described. Firewall? Will try iptables -F.

HOPE someone with deeper IP net knowledge helps.

Have tried iptables -F. No change.
Next step: will compare command line PPP daemon process on working (Fedora 24) and non working (BB 14.07) link.

Again reply to myself (as if someone uses this forum by mail, does not receive notification on EDIT only on REPLY to topic):

  • WORKS, changed SIM (operator) w/o changing APN... That seems to be all :). Now will try to set it up correctly in MWAN3 as "last resort" route to Internet.

The other ("first") SIM does not work either on Linux Fedora laptop, so probably SIM is dead.

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