How to unlock iPhone 4s locked to SoftBank (Japan)?

I have an iPhone 4s (64GB) locked to SoftBank and I tried everything to unlock it but it didn't work, I even purchased r sim 8 gold but it still doesn't work. Please help me find the solution! I studied in Japan but I had to suddenly leave due to health problems and paid full price, now I have a useless expensive phone.
I would really appreciate any help, thank you!

You can go to YouTube and visit this channel - www.youtube.com/icrackuridevice
He had unlocking tutorials and jailbreaks. It will be helpful.

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