I just bought a ZTE Axon Elite 5.5'' 32GB and it works great EXCEPT that the App notifications sometimes don't work in WiFi mode. This includes almost all apps, like LinkedIn, Slack, Tinder, even WhatsApp!! Also, when I use WhatsApp Web it says "Phone not connected" until I manually start the application on the phone. As soon as i switch of WiFi, the messages start flying in. This is fairly annoying, any suggestions?
Check Settings > Data usage > check and make sure app background data is not restricted. Also make sure you have good internet connection, a slow connection may cause the notifications to be delayed.