I recommend that you phone contact your regional D-Link support office and ask for help and information regarding this. Each region handles support differently.
Link> Tech Support Contact Information
We find that phone contact has better immediate results over using email.
As a alternative suggestion, you could install DD-WRT FW on your 890L. For the NA region, this model router is now EOL. So use of 3rd party FW is recommended if stock FW doesn't seem to work out for you.
http://forums.dlink.com/index.php?topic=72749.0
Good Luck.
I don't think D-Link will be able to help with this, because it is unlikely that they are going commit any resources to a firmware fix for a product that (when I had this issue, was not EOL yet) is EOL. Since my router was BNIB as of 12/19, getting an RMA is not going to change anything.
Also their email support is truly hilarious. Don't think I've had more horrible email support from any company.
Certain companies will attempt easy and quick firmware level repairs for such reported bugs on a beta level at least for older models, but from my reading of this forum, Dlink does not do that.
I used recovery mode to flash US firmware at v1.20. Same issue occurred. Based on my research this is a firmware level bug introduced at US 1.09 and it remained unfixed for years. The issue only affects users that have >10 devices on their 2.4 GHz network. I can imagine other Dlink users experiencing the same issue simply chalked it down to the router being old and trashing it.
Issues like this are why I hate firmware engineers with a passion. They always introduce crap updates that break things. For the 890L it makes the router unusable for heavy users because the earlier firmware versions have publicized security vulnerabilities.
I can confirm that this is a firmware level bug, as after switching over to DD-WRT I experienced no such 2.4GHz disconnections despite using largely the same settings for wireless (4 days and counting).
Also, before I tried DD-WRT, setting up my older 868L as an AP and routing all 2.4GHz connections to the 868L (turning off 2.4GHz on the 890L), resulted in no disconnections either.
Amazing how a cheapass old 868L can be more stable than an expensive 890L simply because of firmware bugs.
After my experience with Dlink over the past 1-2 weeks with this crappy issue, and with a failure on Dlink's part to be able to resolve this issue satisfactorily (router is still under warranty), it is safe to say that my next router won't be from Dlink.