Force external display refresh rate
cscreen

You can use the freely available Terminal app cscreen to force display settings. The app is pretty self explanatory.

brew cask install cscreen


cscreen -h
Usage: cscreen [-d &lt;depth&gt;] [-x &lt;width&gt;] [-y &lt;height&gt;] [-r &lt;refresh&gt;] [-s &lt;display&gt;] [-v] [-m] [-f] [-l] [-h]
           [-d &lt;depth&gt;]    : specifies the bit depth (bits per pixel)
           [-x &lt;width&gt;]    : specifies the width in pixels
           [-y &lt;height&gt;]   : specifies the height in pixels
           [-r &lt;refresh&gt;]  : specifies the refresh rate in Hz
           [-s &lt;display&gt;]  : specifies which display to use (defaults to main display)
           use a as the option to -s to specify the action on all displays
           [-i &lt;displayID&gt;]: picks a display based on CGDirectDisplayID (permanent per display)
           continue to use '-s a' for "all displays"
           [-v]            : display valid modes (use -s to specify display or nothing for the default)
           [-m]            : require an exact match
           [-f]            : forces settings (ignores safety mechanisms; USE AT YOUR OWN RISK)
           [-l]            : lists the current displays and modes
           [-p]            : sets the requested display to have the menu bar
           [-h]            : displays the usage

           Note: Using -p will change the display index so you will likely want to use -l again to show the current
           displays if you wish to use -p a second time.