I found the solution: Go to settings in iphone, then go to Mobile, then go to Use Mobile data for, then turn on the button next to ebay app from the app list and this should fix your problem.
I am not sure about android phones but may be you can try finding similar options there such as mobile data and you need to turn on mobile data for ebay app in particular because even your mobile data is on in general still there are options which can be used for restricting using mobile data for the any particular app
