I recently inherited an old 'tired' duplex rental property. I'm trying to borrow against it to fix it up but..?

Maybe the issue with getting the loan is the amount you are trying to borrow. I would suggest that you pick one project at a time to work on in fixing up the property. By breaking up the projects & lowering the amount of the loan needed, you may be able to qualify for such a loan. Once you get that amount paid back, then pick another project to work on & borrow the amount of money needed. By doing it this way it should also help raise your credit score. Hopefully this would make it easier for you to qualify for subsequent loans. Good Luck!