With the MX67C can use use either the built in 4G modem, or an external USB 4G modem, but not both.  If you plug in an external USB 4G modem then it disables the internal 4G modem.   If you want to use two 4G modems then the external one will need to be a 4G "hotspot" style, which an Ethernet connection.  Then you can plug it into "Internet 1".  Note that it will be the primary WAN connection. ... View more