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I have successfully flashed the 520GU per the instructions, but after doing so I cannot get to the OpenWRT shell from the serial port. I canget to the router's OpenWRT shell using the ethernet port though, so I know that the router works. The serial port works too as I can see all the boot messages fly by.

The last boot message I get is:
mini_fo: using storage directory: /jffs

After that, i hit Enter and nothing happens.

Any suggestions?

Based on your description, I think there is a pretty good chance that the serial TX line to the router is shorted to ground or not connected. This is why you can see messages on your PC from the router but it can't hear your response.