RT-AC56U w/ 380_63.2 Not Working with USB WWAN Dongles

[Tamagnun]

Hi everybody,
I'm using a RT-AC56U with Merlin rel. 380_63.2 and the router doesn't work properly with these two USB keys to connect 3G/4G: Huawei K3715 and ZTE K5006Z, both branded by Vodafone Italy network provider, obviously I'm using one Vodafone SIM.
Really, two days ago the K3715 worked well for one night, but in the morning I found it "hanged up", so I tried to reboot the router, but at the end of the boot sequence the router log register always returns the following lines:

Aug  1 02:00:28 kernel: USB Serial support registered for GSM modem (1-port)
Aug  1 02:00:28 kernel: option 2-2:1.0: GSM modem (1-port) converter detected
Aug  1 02:00:28 kernel: usb 2-2: GSM modem (1-port) converter now attached to ttyUSB0
Aug  1 02:00:28 kernel: option 2-2:1.1: GSM modem (1-port) converter detected
Aug  1 02:00:28 kernel: usb 2-2: GSM modem (1-port) converter now attached to ttyUSB1
Aug  1 02:00:28 kernel: usbcore: registered new interface driver option
Aug  1 02:00:28 kernel: option: v0.7.2:USB Driver for GSM modems
Aug  1 02:00:36 WAN Connection: Ethernet link up.
Aug  1 02:00:37 rc_service: wanduck 395:notify_rc restart_wan_if 0
Aug  1 02:00:38 dnsmasq[408]: read /etc/hosts - 5 addresses
Aug  1 02:00:38 dnsmasq[408]: read /etc/hosts.dnsmasq - 2 addresses
Aug  1 02:00:38 dnsmasq-dhcp[408]: read /etc/ethers - 4 addresses
Aug  1 02:00:38 dnsmasq[408]: using nameserver 192.168.5.1#53
Aug  1 02:00:47 WAN Connection: WAN was exceptionally disconnected.

I don't understand why the WAN connection is disconnected, the SIM works properly into a mobile router battery powered (Onda PN51T).
I've also tried to connect to the ASUS router another 3G/4G USB key (ZTE K5006Z, always from Vodafone Italy), but the behaviour at boot time is the same.
With this dongle I've also tried to input the following commands (from another thread), but the udhcpc continously answers "Discovering networks..." and no connection is created:

echo -e "AT+CGDCONT=1,\"web.omnitel.it\"\r" > /dev/ttyUSB0
sleep 5
udhcpc -i usb1 -p /var/run/udhcpc1.pid -s /tmp/udhcpc

May someone help me?
Thank you
Roberto