How do I get a doctor to write a letter for mom's mental incompetency due to dementia?

She is in memory care for 24 hour care and I need to sell her home to help take care of her. Problem is I need two doctors to sign off that she is mentally incompetent. Her primary doctor already wrote a letter for me explaining she can no longer take care of her finances, etc. so I can give it to the title company to sell her home. He had also staged her at moderate severe dementia in 2017.
But, the only other doctor she has seen is a gynecologist. I took a letter to the gynecologist explaining the letter I need him to write (advised by mom's elder attorney), but the nurse said she is sending it instead to administration first. I want to sell her home quickly before a recession. I'm getting frustrated. I don't know if they're going to do the letter now, and don't know how long this is going to take if they do do it.
Should I go to urgent care and have a regular doctor write the letter? Or, make an appointment this week with a geriatric doctor to see her and make the letter? Should I show the gynecologist a copy of the letter the other doctor wrote or show it to a regular doctor so they see it has already been done once?
I don't know what to do since I don't have much time. I already told the realtor I want her home sold fast due to the inversion in the markets two days ago. Plus, I have young children of my own, a full time job, and want to get this taken care of.