Till now 13.4 IOS and problem not solving
Notifications not received for applications like smart home, Facebook, ... etc
issue may happened due to the following
1- iMessage and FaceTime, try to deactivate them as it may be from them
2- if your carrier not support LTE so please work on 3G, check your carrier Apple cooperation together
3- issue happened when you deactivate your WiFi and log into another network WiFi then return again to your first one
here somthing stuck and need time under the network to work automatic again why ?! I don’t know
in some cases not working so we have to reset network and deactivate and activate WiFi many times and reset your phone and router to work
it is seems that your mobile connection stuck Into the WiFi network so when you reconnect again WiFi not see your mobile till you make restart to phone and Router
apple shall know why this happened
may be also because your router have 2 frequency 2.4 ghz and 5 ghz and router is auto so frequency may changed automatic to your mobile and this lead to stuck
if you can fix frequency at your router do it to remove this probability from your calculations
i don’t know exact reason but this is all what I see from many searching
till now IOS 13.4 still the problem exist