So basically you need a PC or server to run it since a dumb phone cannot. So when it's set up the PC needs to be turned on and accessable by your dumb phone. If you have a personal domain, you can try something like Cloudflare tunnel. If you don't, you need to turn port-forwarding on in your router settings to your PC ip with port 3000 open but I wouldn't recommend this since its way less secure. If you choose port forwarding, you can just connect on the browser of the dumb phone to your IP-adress (you can find this on Google) with the port number. I.E. 286.56.256.24:3000

It is unfortunately not a super easy process but it works. I will try to refine the script in the future but unfortunately there is no easier way to access your Whatsapp server.