I want to observe network changes from 4G to 5G use the following code:    if (@available(iOS 12.0, *)) {     [[NSNotificationCenter defaultCenter] addObserverForName:CTServiceRadioAccessTechnologyDidChangeNotification object:nil queue:nil usingBlock:^(NSNotification * _Nonnull note) {     }];   } When I switch my phone's network from 4G to 5G, it do not work。but when I switch my network between 3G and 4G, it did work. when I switch network to 5G, I run CTTelephonyNetworkInfo.new.serviceCurrentRadioAccessTechnology ,it did return CTRadioAccessTechnologyNR which is 5G。 Question is there a bug on CTServiceRadioAccessTechnologyDidChangeNotification when net changes between 4G and 5G? is there a way to know network changes between 4G and 5G? Exclude polling。