You could use SSE (Server Sent Events) and host the service on glitch.com or similar service.
A Look at Server-Sent-Events
Or you could use a Service Worker and FireBase for Push Notifications.
Add push notifications to a web app
The links I provided were picked by a quick search, didn't even read the whole articles. You can further search those topics for more implementation options.
The first option is more appropriate I guess.
I forgot about WebSockets that you already mentioned and that probably is the best candidate I guess since it is bidirectional.
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