Contact the person or company who wrote the code. They usually provide the unencrypted source for a fee or as part of a support agreement.
There is no reliable, 100% free online decoder for modern ionCube files. Most "free" results are either limited previews, outdated scripts, or security risks. online ioncube decoder free
Instead of trying to statically decrypt the file offline, developers run the encoded file on a server with the correct, licensed IonCube Loader installed. They then use PHP debugging extensions (like Xdebug or custom PHP extensions that hook into the compiler) to intercept the "opcodes" (the compiled instructions) after the IonCube loader has decrypted them in memory, but before PHP executes them. These opcodes can then be decompiled back into readable PHP source code. This requires a controlled, local server environment, not a web frontend. Contact the person or company who wrote the code