Turning the power off to your TalkTalk Youview box will make no difference.
The Youview boxes don't currently provide NowTV Entertainment on the NowTV App.
You will need to use a different device to watch your 7 day entertainment trial on, have a read of this link on ways to watch NowTV.
Also make sure you cancel your 7 day free entertainment trial fully by following the approx 4 steps on screen until you see the green thumbs up cancelled message and when going back to My Account > Passes & Vouchers on here, you should see a red cancelled note next to your entertainment pass.
i would cancel the entertainment trial straight away where you will still get the full seven days if you have another device to watch the entertainment trial with.
This doesn't help you in the short term but BT & NowTV have come to a cross supply agreement where NowTV Entertainment will be added to the Youview boxes.
There is no official confirmed date yet, where last i heard it was sometime 2019 and it is still not clear if the agreement covers BT Boxes only or covers the complete Youview platform including Talk Talk and Youview Retail boxes as well.