If i'm right the tx gpio is also a bootcfg gpio and if you have a "strong" pullup resistor in your usb serial adaptor then your device tries to boot not from nand...
Try to connect first only the rx uart gpio, open serial connection on your desktop pc, press key 4 permanently (to enter bootloader command line) and power on your mir3g... After some seconds (>= 10) release key 4 and connect also the tx gpio. Now you should be in the u-boot command line and can also use the reset command to reboot your mir3g or do something else.
As i know, the bootcfg pins are only important on a cold reboot, so the warm reboot should work with connected tx pin.