Uboot upgrade:
--------------

The following worked just without any problem on my wt3020f (8mb).
Download the following archive:
https://mega.co.nz/#!jdRECSxA!tq4kk0zHI6aVbDfXVdNBYf5sXXbu7xw4SsvxoP9gYe4
unzip it and check it:
$ md5sum u-boot-nexx-3020-7620.bin
9002d70c8af2ca878df956934b122a39  u-boot-nexx-3020-7620.bin
offer it via http, so it can be downloaded from the router
$ python -m SimpleHTTPServer

directly connect the router via the lan port to your computer.
$ sudo ifconfig eth0 192.168.8.5
$ telnet 192.168.8.1 (login: nexxadmin pass: y1n2inc.com0755 )
# cd /tmp/
# wget http://192.168.8.5:8000/u-boot-nexx-3020-7620.bin
// proceed only, if you successfully downloaded the new uboot file
# mtd_write unlock mtd0
GetFileSize: No such file or directory
Unlocking mtd0 ...
# mtd_write erase mtd0
GetFileSize: No such file or directory
Unlocking mtd0 ...
Erasing mtd0 ...
Erase char is 255
# mtd_write -r write u-boot-nexx-3020-7620.bin mtd0
Unlocking mtd0 ...
Writing from u-boot-nexx-3020-7620.bin to mtd0 ... [w]

it hangs now, just wait another minute and then power it down and start it up again
To check if the new uboot works, press and hold reset-button, then connect power.
Once the led flashes several times, you can release the reset button.

$ sudo ifconfig eth0 192.168.1.5

On http://192.168.1.1 is the uboot-webinterface
download https://downloads.openwrt.org/snapshots … pgrade.bin
and upgrade it directly from uboot. (Firmware Upgrade -> generic firmware -> firmware)

after a reboot:

$ telnet 192.168.1.1
Trying 192.168.1.1...
Connected to 192.168.1.1.
Escape character is '^]'.
=== IMPORTANT ============================
  Use 'passwd' to set your login password
  this will disable telnet and enable SSH
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Enter 'help' for a list of built-in commands.