Welcome to the forum, @Mr E. Sweet potatoes are a wonderful thing and keep you feeling full for a long time because they are considered a low GI food.
I have been following the 5:2 WOL ( Way Of Life) for nearly three years and, unlike Tracie, have not cut down on eating carbs by much. When I started 5:2 I wanted/needed to make it sustainable for me and so I wasn't prepared to give up potatoes, rice, pasta, cake, biscuits, crackers etc because I enjoy them and didn't want to cut them out of my life forever! However, I have cut down on the amount of them that I eat
on a fast day because there are many other better ways of using my calories. On the other days, well, anything goes - in moderation!
As a man you have more calories to use than we women so if a sweet potato, or any other carby food, sees you through your fast days more comfortably, then go right ahead. Everyone has to make their own way through this WOL and there are as many different ways of doing this as there are members of this community.
Good luck on your 5:2 adventure. It really does work!