Please phone this in and talk to support. I'm sure they can help you. I would ask for escalated support.
The WRT is a temporary solution I would try since this may take longer to fully identify your problem and if it's deemed a FW issue, it would take some time to get it fixed and possible sent to you. So thats why I suggested trying WRT in mean time. However I would wait until you get in contact with D-Link support on the phone as soon as you can.
I've asked D-Link to review this thread.
The WRT FAQ I wrote up yesterday is step by step and if followed, will result in getting the router loaded correctly with WRT and not bricking the router. I didn't brick my 890L.
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It would be helpful to let us know what your network topology is like as well. Help us with figuring out whats wrong. More info we have the better.