To change messages text which cannot be done via backend inactive logout settings page.

functions.php

//Paste this into functions.php file

//For changing message "You have been logged out because of inactivity. Please wait while we redirect you to a certain page..."
add_filter('ina__redirect_message', 'your_theme_function' );
function your_theme_function($msg) {
  $msg = "You have been logged out because of inactivity. Please wait while we redirect you to a certain page. Since, this is a custom message";
  return $msg;
}

//For changing message "You have been logged out because of inactivity."
add_filter('ina__logout_message', 'your_theme_function' );
function your_theme_function($msg) {
  $msg = "You have been logged out because of inactivity. Please wait while we redirect you to a certain page. Since, this is a custom message";
  return $msg;
}