If it's just DNS, as it seems to be from my perspective, that's likely to break the gateway for browsers, but it doesn't mean the gateway is down; just hard to access.
The phone client is resilient to DNS failures, but it's hard to do that for a website used by a browser (I'm not really sure how it would be done, probably something with service workers which can intercept and process outgoing requests?); and impossible to do if you hadn't loaded it previously.
Disclaimer: I worked at WhatsApp until 2019, including on resilience to DNS failures.
Yeah, it's possible to have IP fallbacks in the downloadable app, but if it's not working, it's likely it wasn't done (although I'm not in a place to check if the web gateway is currently working; it's possible two things are broken at the same time)
Looks like it's back now (or in progress to back anyway), but if you want to mess around with your hosts file, and you don't mind a bit of spelunking, you can find your suggested facebook IP with host www.facebook.com, and then look around in the /24 of that IP, looking for whatsapp-cdn-... in reverse DNS. That IP should respond well to requests for web.whatsapp.com. You might need to do something for the websocket hostname(s) too, but I forget the details of that. Might work going to the same IP. Take a look at domains used now, in case you want to do this again in the future.
The phone client is resilient to DNS failures, but it's hard to do that for a website used by a browser (I'm not really sure how it would be done, probably something with service workers which can intercept and process outgoing requests?); and impossible to do if you hadn't loaded it previously.
Disclaimer: I worked at WhatsApp until 2019, including on resilience to DNS failures.