Mobile development

I need an iPhone/iPad app. I already have a design for it, I just need it to be built. Hi. I have an IOS application (both for iPad and iPhone) and I need help fixing some problems. I have buttons on a XIB file and the buttons only work on iPhone 6 Plus and 7 Plus. On the rest of the devices the buttons don't work. In addition, there is a navigation bar, and I would like it to apper and disapper when I tap once on the screen. The tab bar and the buttons are already placed there are just the fixing. In the first view there are 4 buttons, the second upper one should lead to the same XIB but outside of the scrollview (It's very clear once you'll see the project).