If ttyusb missing:

lsub

Bus 002 Device 004: ID 12d1:1465 Huawei Technologies Co., Ltd. K3765 HSPA

but still ttyusb device missing, quick fix:

modprobe usbserial
udevadm trigger

[128729.644381] usb 2-1.3: GSM modem (1-port) converter now attached to ttyUSB0
[128729.644404] option 2-1.3:1.1: GSM modem (1-port) converter detected
[128729.644455] usb 2-1.3: GSM modem (1-port) converter now attached to ttyUSB1
[128729.644476] option 2-1.3:1.2: GSM modem (1-port) converter detected