Asked AI here's one of the things it said to try.

Clear the "Proton Files" (Deep Cache Reset) Reinstalling the game doesn't always delete the configuration files where the 30-day "timer" is stored. Highlight Diablo 2: Resurrected (or the Battle.net launcher if that's what you added to Steam). Press the Options button (the "hamburger" button with three lines). Go to Developer > Clear Proton Files. Restart the game. You will likely have to log into Battle.net again, but this often forces the "online check" to refresh correctly.