EVDO devices (and cellular modems) generally require more power or at least will request it so I'd try first with a powered hub. They also might not work because the driver or hardware isn't quite up to it (I have a Pantech which is cdc_acm and I can do a terminal style connection, but ppp starts up and the usb gives some nasty error messages). /dev/ttyUSBx based EVDO modems may work.