That's not necessarily true. That's a copyright issue. Now if d-link was to say that the product was not abandoned and thus the copyright is still theirs, then you might have a case that they need to fix the issue. That doesn't mean they need to give you the code, but decompiling should be OK. But copyright laws vary quite a bit. So that's a totally separate issue.

But you are welcome to write your own firmware and install it on the device in most localities. You just need write it from scratch, just like replacing a custom gear or motor in a vacuum would require engineering it to fit inside the case and connect with all the appropriate parts. Which you are welcome to do.