I’m not entirely clear exactly what you expect to happen when you press that button but if your can ssh into your 4K+ and run the following and let us know the url it returns it should provide the information needed…
osmc@vero5:~$ cat ~/.kodi/userdata/keymaps/* | paste-log
cat: '/home/osmc/.kodi/userdata/keymaps/*': No such file or directory
https://paste.osmc.tv/epocefavac
osmc@vero5:~$
Yes. You can either install the Keymap Editor add-on and navigate to global>system>suspend and assign that key to it that way or else via ssh again you can type
and then press ctrl+x, then y, then enter, to save the file and exit. You will then be back in the terminal and you can type the following to load the keymap and it should then work.