I had the same problem when i switched from Android to iOS. I have contacted Niu on their facebook account, sent them screenshots from App and phone settings (they told me exactly what they need) + screenshot after a Smart Check. They fixed the problem in about 3 days, all the push notifications are working now. I still have one problem, the push notifications are in chinese but at least they work, i will try to learn some Simplified Chinese now :))))
Hope that helps π