@garry327garry327- Welcome to Mayo Connect Garry. Brain fog from chemo or immunotherapy is very difficult to deal with. At first, with my chemo, I thought that I was becoming loony but read up on it and realized that it's a very "normal" symptom. Even some anti depressives cause mental fog, loss of memory. It usually fades away after you stop the medicine that causes it over a period of time. I hated my fog. Fortunately I had have a very understanding support group.
Otherwise how are yo feeling?
https://www.curetoday.com/publications/canadian-edition/2019/summer-2019/out-of-the-haze-of-chemo-brain
https://www.webmd.com/cancer/ease-cancer-brain-fog