Nedbank refuses to give us out deceased father's investment money

If there was zero star option, that would be the review. My father died last year and left his will with FNB. FNB has been trying to finalise the estate but he left his investment savings in a Nedbank account. Both FNB and his business Auditor has been reaching out to Nedbank to transfer his investment to the estate account but they have literally ignored all emails. FNB even escalated the email to Nedbanks complaints department and STILL NO RESPONSE! FNB even assumed that my mother withdrew the money so my mother had to go to the branch to check if it's still there and it was, they gave us a number to contact cause the branch said they can't help. When we called, my mother was transferred to about 3 people and they kept redirecting her. The last person said that she can't help because they only work via email and that FNB must send another email. FNB still has not received any response. We are all extremely furious with this bank and we demand our fathers money! We do not know what to do at this point.