Huawei E3372 on TL-WDR3600

It's the first time I'm trying to setup a LTE dongle and this is giving me a headache. I was able to set up a Huawei E3372 in stick mode on OpenWrt (18.06-SNAPSHOT r6909) running on a x86_64 machine by following this thread: Huawei E3372 with NCM

The following configuration works on the x86 machine, the modem initializes and I get an address for device wwan0:

config interface 'wwan'
	option proto 'ncm'
	option ifname 'wwan0'
	option apn 'internet.telekom'
	option pincode '0000'
	option username 'tm'
	option password 'tm'
	option dialnumber '*99#'
	option ipv6 'auto'
	option device '/dev/ttyUSB0'
	option mode 'preferlte'

With the exact same LTE stick on a TL-WDR3600 with the same USB/NCM packages compiled from the same snapshot I'm not able to connect though. The modem seems to initialize fine, but udhcpc is waiting forever for an address.

Tue May 15 15:34:42 2018 kern.info kernel: [   83.473799] usb 1-1.1: new high-speed USB device number 5 using ehci-platform
Tue May 15 15:34:42 2018 kern.info kernel: [   83.627735] hub 1-1.1:1.0: USB hub found
Tue May 15 15:34:42 2018 kern.info kernel: [   83.632398] hub 1-1.1:1.0: 4 ports detected
Tue May 15 15:34:51 2018 kern.notice kernel: [   92.735833] random: crng init done
Tue May 15 15:35:02 2018 kern.info kernel: [  103.443782] usb 1-1.1.4: new high-speed USB device number 6 using ehci-platform
Tue May 15 15:35:03 2018 kern.info kernel: [  104.169829] usb 1-1.1.4: USB disconnect, device number 6
Tue May 15 15:35:04 2018 kern.info kernel: [  104.973779] usb 1-1.1.4: new high-speed USB device number 7 using ehci-platform
Tue May 15 15:35:04 2018 kern.info kernel: [  105.190212] option 1-1.1.4:1.0: GSM modem (1-port) converter detected
Tue May 15 15:35:04 2018 kern.info kernel: [  105.197125] usb 1-1.1.4: GSM modem (1-port) converter now attached to ttyUSB0
Tue May 15 15:35:04 2018 kern.info kernel: [  105.204843] option 1-1.1.4:1.1: GSM modem (1-port) converter detected
Tue May 15 15:35:04 2018 kern.info kernel: [  105.211687] usb 1-1.1.4: GSM modem (1-port) converter now attached to ttyUSB1
Tue May 15 15:35:04 2018 kern.info kernel: [  105.365089] huawei_cdc_ncm 1-1.1.4:1.2: MAC-Address: 00:1e:10:1f:00:00
Tue May 15 15:35:04 2018 kern.info kernel: [  105.371732] huawei_cdc_ncm 1-1.1.4:1.2: setting rx_max = 16384
Tue May 15 15:35:04 2018 kern.info kernel: [  105.431135] huawei_cdc_ncm 1-1.1.4:1.2: NDP will be placed at end of frame for this device.
Tue May 15 15:35:04 2018 kern.info kernel: [  105.439925] huawei_cdc_ncm 1-1.1.4:1.2: cdc-wdm0: USB WDM device
Tue May 15 15:35:04 2018 kern.info kernel: [  105.447313] huawei_cdc_ncm 1-1.1.4:1.2 wwan0: register 'huawei_cdc_ncm' at usb-ehci-platform-1.1.4, Huawei CDC NCM device, 00:1e:10:1f:00:00
Tue May 15 15:35:05 2018 daemon.notice netifd: Interface 'wwan' is setting up now
Tue May 15 15:35:07 2018 daemon.notice netifd: wwan (2209): sending -> AT
Tue May 15 15:35:08 2018 daemon.notice netifd: wwan (2209): sending -> ATZ
Tue May 15 15:35:09 2018 daemon.notice netifd: wwan (2209): sending -> ATQ0
Tue May 15 15:35:09 2018 daemon.notice netifd: wwan (2209): sending -> ATV1
Tue May 15 15:35:10 2018 daemon.notice netifd: wwan (2209): sending -> ATE1
Tue May 15 15:35:10 2018 daemon.notice netifd: wwan (2209): sending -> ATS0=0
Tue May 15 15:35:11 2018 daemon.notice netifd: wwan (2209): sending -> AT+CGDCONT=1,"IP","internet.telekom"
Tue May 15 15:35:15 2018 daemon.notice netifd: wwan (2209): Trying to set PIN
Tue May 15 15:35:16 2018 daemon.notice netifd: wwan (2209): PIN set successfully
Tue May 15 15:35:16 2018 daemon.notice netifd: wwan (2209): Configuring modem
Tue May 15 15:35:16 2018 daemon.notice netifd: wwan (2209): Setting mode
Tue May 15 15:35:16 2018 daemon.notice netifd: wwan (2209): sending -> AT^SYSCFGEX="030201",3fffffff,2,4,7fffffffffffffff,,
Tue May 15 15:35:18 2018 daemon.notice netifd: wwan (2209): Starting network wwan
Tue May 15 15:35:18 2018 daemon.notice netifd: wwan (2209): Connecting modem
Tue May 15 15:35:18 2018 daemon.notice netifd: wwan (2209): sending -> AT^NDISDUP=1,1,"internet.telekom","tm","tm"
Tue May 15 15:35:19 2018 daemon.notice netifd: wwan (2209): Setting up wwan0
Tue May 15 15:35:19 2018 daemon.notice netifd: Interface 'wwan_4' is enabled
Tue May 15 15:35:19 2018 daemon.notice netifd: Interface 'wwan' is now up
Tue May 15 15:35:19 2018 daemon.notice netifd: Network device 'wwan0' link is up
Tue May 15 15:35:19 2018 daemon.notice netifd: Network alias 'wwan0' link is up
Tue May 15 15:35:19 2018 daemon.notice netifd: Interface 'wwan_4' has link connectivity
Tue May 15 15:35:19 2018 daemon.notice netifd: Interface 'wwan_4' is setting up now
Tue May 15 15:35:19 2018 daemon.notice netifd: wwan_4 (2390): udhcpc: started, v1.28.3
Tue May 15 15:35:19 2018 daemon.notice netifd: wwan_4 (2390): udhcpc: sending discover
Tue May 15 15:35:22 2018 daemon.notice netifd: wwan_4 (2390): udhcpc: sending discover
Tue May 15 15:35:25 2018 daemon.notice netifd: wwan_4 (2390): udhcpc: sending discover

Does anyone have an idea what's causing this? The only real difference is kernel 4.14 vs 4.9 between these targets, but for a kernel issue I would expect an error message in dmesg.
I'm thankful for any pointer on how to debug this.

I haven't had much time to investigate my E3372 problem, especially since each attempt crashes my primary HH5a router, but glad to hear you have it working on x86. When I have time, I think I'll drag out my previous hardware (a Netgear WNDR3800) and see how the dongle performs there ...