There are no new Sky+ boxes. They haven't been manufactured since 2015/2016. Most likely the box is faulty, or the cable(s) from the dish isn't fitted quite right - it's very easy to bend the centre 'pin' of the connector if it's not lined up with the socket when pushing it in.
It's conceivable that there's a fault elsewhere with your dish setup, but that would seem unlikely if your previous Sky+ box was working ok.
If you previously had a Sky Q box the dish will have been altered to accommodate it, and is no longer compatible with Sky+.
