"Google Messages uses the phone as the gateway. The web app sends the messages to and receives the messages from the phone. If the phone is off this does not work. However the phone does not need to be nearby just on and connected to the internet and in a cell coverage area."
If you're already using Google Messages, the you can use their web app from any pc browser, https://messages.google.com/web/authentication I've a Google Voice acct that I use to text from my pc browser. One advantage of it over the Google Messages web app is that I don't have to constantly re-link my phone as my phone is usually always in airplane mode unless I've a specific need to use it.
