Question Retargetting problem

So Unity retargetting system has been doing this every time i switch the character model. I've tried adjusting the t-pose, messing around with the muscle limits but every single time the hands are off place.

To note that the characters have different rigs. The old one (also used during the animation process) was created in Blender and the new character has a rig made in mixamo.

Any tips on how to solve this without breaking the animations will be greatly appreciated!

Thanks.

Old character model


New character model