Firstly, I'm going to assume that you are subscribed to TNT Sports through SKY (as in you are paying SKY for TNT sports). If that's the case, then I had the exact same issue. On the MySky app, it said it was a premium subscription, but I could not access the TNT Sports channels on the discovery plus app. It was stuck on basic. It's no exaggeration, I spoke to 5 or 6 sky advisors who asked me to try several things with my sky Q box, none of which worked. The last thing they suggested was to remove TNT Sports from my subscription for 24 hours and then re-add it. This worked, basic switched to premium and I could see all the TNT Sports channels on the discovery+ app. Hope this helps.
As an aside, while i was stuck on basic discovery I could still watch the tnt sports channels on sky go app. I just couldn't watch them on the discovery+ app. But obviously it needed to be sorted because the discovery+ app is far superior. I was incredibly relieved when it was finally fixed by removing and re-adding TNT after 24 hours.
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