I cannot reproduce the issue on Ubuntu PC.
do you see something like the following in dmesg which will deregister the eth interface?
[15621.974691] GobiNet 3-1:1.8 eth1: unregister ‘GobiNet’ usb-0000:00:14.0-1, GobiNet Ethernet Device
[15382.348289] usb 3-1: New USB device found, idVendor=1199, idProduct=9071
[15382.348300] usb 3-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[15382.348307] usb 3-1: Product: Sierra Wireless MC7430 Qualcomm\xffffffc2\xffffffae\xffffffae Snapdragon\xffffffe2\xffffff84\xffffffa2\xffffff84\xffffffa2 X7 LTE-A
[15382.348313] usb 3-1: Manufacturer: Sierra Wireless, Incorporated
[15382.348318] usb 3-1: SerialNumber: LU62630017041012
[15382.349966] GobiSerial 3-1:1.0: GobiSerial converter detected
[15382.350261] usb 3-1: GobiSerial converter now attached to ttyUSB0
[15382.351210] GobiSerial 3-1:1.3: GobiSerial converter detected
[15382.351506] usb 3-1: GobiSerial converter now attached to ttyUSB1
[15382.352606] GobiNet 3-1:1.8 eth1: register ‘GobiNet’ at usb-0000:00:14.0-1, GobiNet Ethernet Device, c6:f9:56:d8:1b:fb
[15382.353040] USB Speed : USB 2.0
[15382.354108] GobiNet 3-1:1.10 eth2: register ‘GobiNet’ at usb-0000:00:14.0-1, GobiNet Ethernet Device, c6:f9:56:d8:1b:fb
[15382.354648] USB Speed : USB 2.0
[15394.425445] creating qcqmi0
[15394.425693] RawIP mode
[15394.517512] creating qcqmi1
[15394.517760] RawIP mode
//here I type
//sudo echo -ne “AT\x21RESET\xd\xa” > /dev/ttyUSB1
[15621.974072] usb 3-1: USB disconnect, device number 44
[15621.974226] GobiSerial driver ttyUSB0: GobiSerial converter now disconnected from ttyUSB0
[15621.974238] GobiSerial 3-1:1.0: device disconnected
[15621.974594] GobiSerial driver ttyUSB1: GobiSerial converter now disconnected from ttyUSB1
[15621.974606] GobiSerial 3-1:1.3: device disconnected
[15621.974691] GobiNet 3-1:1.8 eth1: unregister ‘GobiNet’ usb-0000:00:14.0-1, GobiNet Ethernet Device
[15622.024905] GobiNet 3-1:1.10 eth2: unregister ‘GobiNet’ usb-0000:00:14.0-1, GobiNet Ethernet Device
[15629.461218] usb 3-1: new high-speed USB device number 45 using xhci_hcd
[15629.478449] usb 3-1: config 1 has an invalid interface number: 8 but max is 3
[15629.478459] usb 3-1: config 1 has an invalid interface number: 10 but max is 3
[15629.478465] usb 3-1: config 1 has no interface number 1
[15629.478471] usb 3-1: config 1 has no interface number 2
[15629.479220] usb 3-1: New USB device found, idVendor=1199, idProduct=9071
[15629.479227] usb 3-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[15629.479234] usb 3-1: Product: Sierra Wireless MC7430 Qualcomm\xffffffc2\xffffffae\xffffffae Snapdragon\xffffffe2\xffffff84\xffffffa2\xffffff84\xffffffa2 X7 LTE-A
[15629.479240] usb 3-1: Manufacturer: Sierra Wireless, Incorporated
[15629.479245] usb 3-1: SerialNumber: LU62630017041012
[15629.480866] GobiSerial 3-1:1.0: GobiSerial converter detected
[15629.481256] usb 3-1: GobiSerial converter now attached to ttyUSB0
[15629.481988] GobiSerial 3-1:1.3: GobiSerial converter detected
[15629.482238] usb 3-1: GobiSerial converter now attached to ttyUSB1
[15629.483295] GobiNet 3-1:1.8 eth1: register ‘GobiNet’ at usb-0000:00:14.0-1, GobiNet Ethernet Device, c6:f9:56:d8:1b:fb
[15629.483822] USB Speed : USB 2.0
[15629.484795] GobiNet 3-1:1.10 eth2: register ‘GobiNet’ at usb-0000:00:14.0-1, GobiNet Ethernet Device, c6:f9:56:d8:1b:fb
[15629.485446] USB Speed : USB 2.0
[15641.548615] creating qcqmi0
[15641.548739] RawIP mode
[15641.552681] creating qcqmi1
[15641.552736] RawIP mode