One thing that you should always do, in case you need to revert to the previous firmware, is make a backup of your configuration. I have a manual for your router, but it's on my other PC, so I can't refer to it right now. But hopefully, you did do that, right?
click here and download one of the earlier listed firmware revisions. Install it and with luck, you will find that it will work and you will only need to configure it again. First perform a factory rest from the UI if you can.