iOS Detect 3G or WiFi

Import Apple's Reachability and try this,

#import "Reachability.h"
#import <CoreTelephony/CTTelephonyNetworkInfo.h>

//Try this
Reachability *reachability = [Reachability reachabilityForInternetConnection];
    [reachability startNotifier];

    NetworkStatus status = [reachability currentReachabilityStatus];

    if(status == NotReachable)
    {
       NSLog(@"none");
        //No internet
    }
    else if (status == ReachableViaWiFi)
    {
        NSLog(@"Wifi");
        //WiFi
    }
    else if (status == ReachableViaWWAN)
    {
        NSLog(@"WWAN");


    //connection type
    CTTelephonyNetworkInfo *netinfo = [[CTTelephonyNetworkInfo alloc] init];
    _carrier = [[netinfo subscriberCellularProvider] carrierName];

    if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyGPRS]) {
        NSLog(@"2G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyEdge]) {
        NSLog(@"2G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyWCDMA]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyHSDPA]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyHSUPA]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMA1x]) {
        NSLog(@"2G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMAEVDORev0]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMAEVDORevA]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyCDMAEVDORevB]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyeHRPD]) {
        NSLog(@"3G");
    } else if ([netinfo.currentRadioAccessTechnology isEqualToString:CTRadioAccessTechnologyLTE]) {
        NSLog(@"4G");
    }

    }

References(Links may broke in future) :

• iOS Detect 3G or WiFi
• Detect carrier connection type (3G / EDGE / GPRS)
  
• What are the differences between 1G, 2G, 3G, 4G and 5G?
  
• CDMA2000
  
• wiki 2G
  

• What is the difference between CDMA and WCDMA network sim
  

• What is "EV-DO Rev. A"?

Using the code that Apple has provided here

Reachability *reachability = [Reachability reachabilityForInternetConnection];
[reachability startNotifier];

NetworkStatus status = [reachability currentReachabilityStatus];

if(status == NotReachable) 
{
    //No internet
}
else if (status == ReachableViaWiFi)
{
    //WiFi
}
else if (status == ReachableViaWWAN) 
{
    //3G
}

When using iOS 12 or newer, you can use NWPathMonitor instead of the pre-historic Reachabilityclass :

import Network // Put this on top of your class

let monitor = NWPathMonitor()

monitor.pathUpdateHandler = { path in
    if path.status != .satisfied {
        // Not connected
    }
    else if path.usesInterfaceType(.cellular) {
        // Cellular 3/4/5g connection
    }
    else if path.usesInterfaceType(.wifi) {
        // Wi-fi connection
    }
    else if path.usesInterfaceType(.wiredEthernet) {
        // Ethernet connection
    }
}

monitor.start(queue: DispatchQueue.global(qos: .background))

If you don't want to import Reachability library or deal with notifiers, you can use this simple synchronous method:

typedef enum {
    ConnectionTypeUnknown,
    ConnectionTypeNone,
    ConnectionType3G,
    ConnectionTypeWiFi
} ConnectionType;


+ (ConnectionType)connectionType
{
    SCNetworkReachabilityRef reachability = SCNetworkReachabilityCreateWithName(NULL, "8.8.8.8");
    SCNetworkReachabilityFlags flags;
    BOOL success = SCNetworkReachabilityGetFlags(reachability, &flags);
    CFRelease(reachability);
    if (!success) {
        return ConnectionTypeUnknown;
    }
    BOOL isReachable = ((flags & kSCNetworkReachabilityFlagsReachable) != 0);
    BOOL needsConnection = ((flags & kSCNetworkReachabilityFlagsConnectionRequired) != 0);
    BOOL isNetworkReachable = (isReachable && !needsConnection);

    if (!isNetworkReachable) {
        return ConnectionTypeNone;
    } else if ((flags & kSCNetworkReachabilityFlagsIsWWAN) != 0) {
        return ConnectionType3G;
    } else {
        return ConnectionTypeWiFi;
    }
}