@WillBetteM,
Adding the ATS exception would only affect the incoming traffic to your application (for example, an ad being served from the server to your app). As an FYI, iOS 10 would disable the ability to add this exception.
Coming back to the original issue, iOS has a strict mixed content policy, so if any traffic is served on HTTP, there might be an error. If you have control of the clickthrough URL (if it is exposed), you might simply stitch an s to the protocol before you invoke the clickthrough. Since UIWebView disables allowing mixed content, the best bet is to have the vendor enable HTTPS on their end as well.
Vu