using an unicode code in a latex-document

I am writing a danish document. I want to use an old glyph unicode no: U+2184 [or U+0254, U+0186]. I do not succeed. Why and how should I proceed?

% !TEX TS-program = xelatex
% !TEX encoding = UTF-8
\documentclass{memoir}
\begin{document}
Der var en snes {\char"2184}: 20 stk.
\end{document}

The tex code you have is working, you just need to choose a font which has this letter.

To find out which of the fonts on your computer have it, you can run

albatross ↄ

in a terminal and you'll get a list. For example with Arial:

% !TEX TS-program = xelatex
% !TEX encoding = UTF-8
\documentclass{memoir}

\usepackage{fontspec}
\setmainfont{Arial}

\begin{document}
Der var en snes {\char"2184}: 20 stk.
\end{document}