I'm so relieved to find this thread as I've been googling this issue for two days to no avail. My WhatsApp push notifications suddenly stopped working yesterday, and the desktop app will only work when I have WhatsApp actively open on my phone, showing on the screen. If I don't do that it says "phone not connected."
I have a 7 that's (now) running 13.4.1. I updated yesterday in trying to fix this issue. Other things I have tried are: Updated WhatsApp to the latest version. Restarted the phone. Made sure background refresh is on. Made sure notifications are on both in Settings and in the app. Made sure DND is not on. Turned off and on wifi. Turned off and on cellular. Logged out of and back into the desktop app. Deleted, re-downloaded and reinstalled the desktop app. Reset network settings. Reset ALL settings. Really the only thing I haven't done is deleted and reinstalled WhatsApp on my phone, and I'm very hesitant to do that because I think it will erase all my chat history.
I'm relieved that this is at least a known issue, but I'm extremely bummed to see that it's been a full 6 months and there's no fix. ESPECIALLY now in lockdown I chat with my friends a lot and it's making me crazy to have to manually open the app and check if there are any messages. I keep missing stuff and it stinks. I also use the desktop app the majority of the time and it's basically useless now.
Thanks for being there, other 462 of you who have this question too.