NSTimer, wont start when I go back to the ViewController

I have a NSTimer which is making an HTTP Request every 25 seconds. This works fine, but when I go to another ViewController and then back. It Does not work probably. What am I doing wrong?

-(void)viewDidAppear:(BOOL)animated {

    timer = [NSTimer timerWithTimeInterval:26.0 target:self selector:@selector(sendRequestToGetData:) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:timer forMode:NSDefaultRunLoopMode];

}

NSTimer

-(void)sendRequestToGetData:(NSTimer *)timer
{


NSDictionary *headers = @{},
*parameters = @{};

UNIHTTPJsonResponse *response = [[UNIRest get:
                                  ^(UNISimpleRequest* request) {
                                      [request setUrl:
                                       @"http://localhost/getfixture.php/fixtures"
                                       ];
                                      
                                      [request setHeaders:headers];
                                      [request setParameters:parameters];
                                  }] asJson];

NSData* rawBody = [response rawBody];
jsonResult = [NSJSONSerialization JSONObjectWithData: rawBody
                                             options: NSJSONReadingMutableContainers
                                               error: nil];

}

Try this :

NSTimer *myTimer; 

-(void)viewDidAppear:(BOOL)animated {    
   myTimer = [NSTimer scheduledTimerWithTimeInterval:26.0
        target:self
        selector:@selector(sendRequestToGetData:)
        userInfo:nil
        repeats:YES];
}

- (void)viewDidDisappear:(BOOL)animated{
   [myTimer invalidate];
   myTimer=nil;
}