Our adopted dog has extreme separation anxiety and confinement anxiety, help!

We recently adopted a two year old dachshund mix and he is a well tempered dog. The problem is that when we try leaving him in his crate he will bark non stop, without stopping for hours. He goes as far as tearing his blanket up and being destructive. Now, when we tell him to go to the crate he willingly does and will actually stay in in AS LONG as the door stays open. We are also trying the tethering method but he still goes wild and barks nonstop for that aswell. He gets exercise and plenty of potty breaks throughout the day so it’s just so frustrating. I should also not that he cage is right next to our bed so I’m not even sure why he barks when we are right there. He needs to be in the crate whenever no one is home because he will pee wherever. Please any advice is so very appreciated.