Linking text boxes has nothing to do with lines. It has to do with having text flow from one to another. Word jargon, sorry.

Let's start without the text boxes. Can you draw a line?
Insert > Shapes > Lines

If that works for you, then with two text boxes on your screen, draw a line of the approximate length and direction that you want without touching any text box. Then move it to where it touches one of the text boxes. If needed, resize the line so it touches the other.

To me, this is relatively straightforward, yes.

By the way, I'm a lawyer and seldom use text boxes or graphic lines.