I was doing on ar300m onetime this week too.
Update to openwrt-ar300m-3.201-0402, didnt work by me too. Doing in two stepps was working:
- reset to factory
- update by hand to oldest of newer firmware one, can be 3.109 or so on ???
- after than updated by hand to openwrt-ar300m-3.201-0402
- A it can be this version have a DNS leak, so it can be its better to use 3.103; 3.104 or 3.105 or so on
It can be I was useing allways the .tar version.