Money lost due to error on Nedbank app

I opened an investment on the Nedbank app. 3 months later, the interest payable was reduced by 0.85%. I lodged a complaint, and after 2.5 weeks, I was referred to terms and conditions, where the quoted interest rate is only valid for 3 months. However, the account was opened on the Nedbank app, on which the terms and conditions make no mention that the preferential interest rate is only valid for 3 months. I spent an hour on.the phone with the Nedbank call centre, and have sent numerous emails to the complaints department. However, they will not admit the error. Clients must just know that they should first hunt through all terms and conditions on the Nedbank website, before they do anything on the Nedbank app.