This is the climax of the story, and it goes one of two ways:
Once you have downloaded the .bin or .img firmware file from the official link, follow these steps to install it: dsl3890 firmware link
The "story" of the DSL-3890 firmware link is not a simple tale of a download button. It is a drama of internet service providers (ISPs), locked-down hardware, and the eternal struggle between corporate security policies and the end-user’s desire for control. This is the climax of the story, and
They type "DSL-3890" into the search bar, hit enter, and... nothing. Or, they find a page that refuses to show a "Download" button. and... nothing. Or