yes, had the same issue
but that means we are going through NAT 2 times, once with with the router, once with the usb key. this is why the key appears as tethering/eth and not usb in the interface.
the old 3372 could be changed to stick/modem mode by applying the stick firmware instead of the hi link (nat/router) firmware
in this case you would not need to change the subnet, and the performance would be better.
but the latest version 3372-320 seems more difficult to switch to stick mode/firmware. i am still searching how to do it.