THIS WORKED. I had iTunes Radio playing for an hour or so, and I never got the ad. About 5 minutes into playing this game, it popped up at the bottom of my screen. I clicked it, and I actually got the 5 dollars. Awesome.
I got it to come up, however it says I already received the gift card. It says click here to see how to redeem it. It then takes me to the App Store and shows a thing for Verizon saying today is connection day. I don't know what to do or go to get the code. Awesome, I got it to work, I closed all of m apps and tried again and it worked! Thank you New England gamer!!!
I think it might be able to detect if you've already redeemed it on your network. I was on wifi and redeemed it. I tried to do it for my wife's account, but it was giving me the already redeemed message. So I had to go off of cellular data and it allowed me to redeem it. If others don't have a game with iAds on it readily available, here's the game I used: Hungry Cat Picross by Tuesday Quest https://appsto.re/us/zzr-R.i
The easiest way to get the credit is to download Daddy Long Legs open the app and if it doesn't show up when the iAds show close the app and then open them again, after a few times it should pop up.
Are you at least getting a the iAds? If they are not showing up, you might be jailbroken or have some kind of ad block that is preventing it from working.
Maybe you already have it redeemed and you have to sign in again, worked for me. But who knows, maybe they are officially out of vouchers?