Why sharing amongst different devices is not working at all?

Plex Players Mobile Devices
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I have a Synology NAS with Plex.
I use Iphone 12 v 15.x and an Ipad mini v 15.x and purchased an activation on the Iphone. The issue is that playback of my movies on the Ipad stops every 60 seconds. I use the same login names on both devices. Any Idea? It looks play back is working on my PC, on the IPhone but it is not working on the Ipad. It stops playing back the movie and offers activation, but then even the activation button is inactive!

Update? now I can purchase an activation on my Ipad as well… Would Apple reimburse me if I pay for the same application with the same account twice?
Update2: problem solved: the app recognized it and now it works

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Which kind of login? That of Plex? That’s not significant in this case.

For the activation to be used by all your devices, these devices must be signed into the same Apple ID. https://support.plex.tv/articles/205556278-unlocking-or-activating-plex-for-ios/