diamond membership doesn’t work


In post 2 you said you did the trial on this account as well. If you purchased through the app, a staff member has to add the membership to the account.
That said, you will be charged on both now, so you'll probably want to write a ticket to support and close one of the accounts.
https://support.chess.com/customer/portal/emails/new


Memberships on the app are tied to the store app account ID, not the device. I don't know how it exactly works for linking though.
You will need to contact staff to resolve.