Did you try a tax/estate lawyer?
Even if your Mom avoided probate, the executor can sometimes open an estate account, I believe. After all, the check is addressed to the estate; it’s part of the estate; and the executor is empowered to take actions to distribute the proceeds of the estate.
I am going through a similar situation as executor. I bank with Chase and I had no problems, however you need a tax id to open an estate account. Easy to get one with a phone call. Hope this helps