You can't, you're dead. Whomever you willed your estate to will lay claim to those accounts. Even if they're secret. What ever institution holding those accounts will claim them for themselves. There's no legal precedent for a person returning from the dead.
Yeah, the idea is to do it before you die. You could e.g. setup a foundation to operate after your death and have in its statue (article of incorporation?) that if you ever return you get them back.
Besides technical solutions, like using cryptocurrency or simply burrying your assets somewhere only you know about where they'd be safe. Or use this, if those announcing it got anywhere and weren't scamers:
The Reanimation Foundation will, it is said, be the "first cryonics
organization to enable individuals signed up for cryonics to take their
money with them. It will be able to offer this service because it will be incorporated in Liechtenstein, which has no rule against perpetuities."
>There's no legal precedent for a person returning from the dead.
How about people being declared legally dead after gone missing and then re-appearing a few decades later. There have been several such cases in the past (sometimes those people got married elsewhere and started a new life, etc).