Watch both the USB cable and the Wifi - - -
See if uboot is issuing discovery commands to load a network-boot kernel.
(It **should**, but there is never any telling what the crew at lab126 has changed.)
I.E: This is going to be a lot easier to fix running under Linux than running under uboot.
Unlike some menu entry labels - "erase MMC" does exactly that.
(Plus, in some firmware versions, re-installs uboot.)
Sure do hope you have a copy of the "Device and Owner Specific" data stored in the eMMC outside of the file systems.
Just getting it to boot again is only the start of your problems.
Meanwhile - start looking for the contact information of Amazon Customer Service. You may be returning this device as "broke" for exchange.