InDesign CC: How do you add a font to the library?

Dear Everyone and Anyone Who Can Help:

I'm using InDesign via Creative Cloud with Windows 10. I'm editing a preexisting document, which I've saved as a new document. When I open the newly saved document, it says the fonts are missing -- which is funny to me, since InD has no problem locating those fonts when I open the older (original) document.

The font family in question is Univers LT Std. I've downloaded the the entire family and added it to the font library on my PC, but InD doesn't recognize/find it. I copied the family into the Fonts folder of the Adobe InDesign 2021 folder in my Program Files. Still not found when I try to substitute the fonts in the document.

From what I gather in other posts here, this font is no longer supported by Adobe -- whatever. But as I have already downloaded the font family, where do I put it so that InD finds it?

Really hoping someone can help. I'm only semi-literate in the back-end workings of computers and where things are located, so I'm *really* hoping there's a simple fix to this.

MANY thanks in advance for any advice!