Did you try with a built-in creep law too ? Did you try different values of the *VISCO, CETOL parameter ? You can find some tips for the built-in creep model here: Creep calculation and results
I have bad experience when it comes to convergence with Mortar contact, but sometimes it may work better than penalty contact.
Your model may have initial rigid body motions and is loaded with force which is bad for convergence. It would be best to remove advanced features one by one to see exactly what causes convergence issues.