26.3, 28; Set Default Font... CMU Typewriter Text

[VanL]

Hello,

I am unable to set the default font to CMU Typewriter Text.  The
menu-bar-options list this font as available.  My .Xresources has

  Emacs.FontBackend: ftcrhb,x

The messages buffer lists "set-face-attribute: Font not available:"

┌────
│ ;; GNU Emacs 26.3 (build 1, i386-unknown-openbsd, 
│ ;;                          GTK+ Version 3.24.12)
│ ;;  of 2019-10-11
│ set-face-attribute: Font not available: #<font-spec xft nil 
│ CMU\ Typewriter\ Text nil nil semi-bold normal nil 10.0 nil nil nil 
│ ((:name . "CMU Typewriter Text"))>
│ 
│ ;; GNU Emacs 28.0.50 (build 2, armv7l-unknown-linux-gnueabihf, 
│ ;;                             GTK+ Version 3.24.5)
│ ;;  of 2020-01-05
│ set-face-attribute: Font not available: #<font-spec nil nil 
│ CMU\ Typewriter\ Text nil nil semi-bold normal nil 10.0 nil nil nil nil>
└────

The font was installed from

  https://www.checkmyworking.com/cm-web-fonts/Computer%20Modern.zip

The font is recognised by

  % fc-list | grep -i cmu

showing 

  # openbsd
  /usr/local/share/fonts/cmu/cmuntt.ttf: CMU Typewriter Text:style=Regular

  # linux
  /usr/share/fonts/truetype/cmu/cmuntt.ttf: CMU Typewriter Text:style=Regular

What's missing?

-- VL