The most reliable method for older systems is using your browser.
Since the native app is a no-go, here are the most reliable ways to stay connected: WhatsApp Web (Recommended) : This is the most stable method. Simply open WhatsApp Web in a modern browser. : For the best experience on 10.13.6, use Firefox (version 115) , which is the final version supported on High Sierra. The "Pseudo-App" Trick whatsapp for macos 10136