third party app not working

I am having trouble with a third party app called tramtracker. When i am not connected to wifi it will work sometimes and sometimes it wont. When i am connected to wifi it will work. all the time. All my other apps and internet connection work. I have downloaded the latest update and still get the same problem. I have had an iphone for the past 6 years and this is the first time i have had this problem. When it dosent work it says unable to contact the tram tracker service. Please make sure your device is connected to the internet and try again. Any suggestions on how to get it to work properly.