Unfortunately if you've unlinked your accounts then nothing can be done and you'll need to wait out the 90 day penalty.
The 90 day cooldown is part of Bungie's cross save policy and as such can't be reduced or removed.
[url=https://www.bungie.net/7/en/CrossSave]Cross Save FAQ[/url]
[quote]Can I disable Cross Save once I’ve enabled it?
Cross Save is intended to be a one-time action, but we realize there may be cases in which you want to visit the characters you left behind. Once Cross Save has been disabled, you will have access to your original characters on each platform. [b]There is a 90-day wait to re-enable the feature. This policy exists to discourage activity like account recoveries by other players.[/b][/quote]