HBO Now keeps asking me to renew my subscription??

I just signed up for the free trial of HBO now, so that I can watch the new episodes of Game of Thrones - Season 6.

I downloaded the app on my iPhone 5s, and connected it up to my Apple ID. I am on the free trial.

Every time I open the app (or the desktop website) I get a page telling me to 'Restart NOW for Unlimited Access, restart your subscription to begin streaming all of HBO.'

When I select 'Renew My Subscription' I get a loading screen, and then a moment later a screen pops up which says 'Welcome back to HBO NOW' with the option to 'Start Watching'. Clearly this makes me think it's worked. So I tap 'Start Watching', and navigate to the episode I want to watch. I tap on it as if to start playing it, but I get the same screen which tells me I need to Renew my Subscription. If I follow the on-screen prompts I will end up in the same situation again.

There is another option which allows me to 'Link your new subscription'. When I click that I'm prompted to select the provider I bought my subscription through (iTunes) and then I enter my email and password, and click 'Sign In'. This takes me right back to the screen telling me to Renew my Subscription.

Basically, I can't watch anything. HBO NOW seems to think my account needs renewing, but I've only just created it. I tried managing my subscriptions via the iTunes app to turn off auto-renewal, but this doesn't make a difference. I don't know what to do.

Can anyone please help? I'm really confused.