you can use something like Autorig Pro to retarget the bones. The bones have to be the same or the actions from Mixamo cannot be applied.
If that's not something you can do, you could delete the skeleton from the target model, copy the skeleton from the mixamo rig, and use weight paint to affix your mesh target to the mixamo rig.
Then go into the Actions dropdown in the "Blend" window of the properties tab at the top right, select the mixamo action and paste it into the non-linear animation timeline. Once it's there, you have to "push it down" to turn it into an animation the new target character can use.
I'm not going to lie--this isn't an easy-to-follow process, but it does work.