I had the same issue. Adding a lot of water helped but I still haven’t been normal. Today was an eating day but I wasn’t really hungry, so I made a mint “milkshake” with an ice cold can of full fat coconut cream, some peppermint extract, some allulose and some ice cubes. I also just got magnesium citrate today and took 400mg. Between that and the super high fat drink I have, ahem, fixed that problem. Good luck!
Could you add a little prune juice to your eating window to keep you regular? It doesn’t taste great but it is pretty effective stuff.
Cant tell from your title if it means you are 12 days into doing fasting, or 12 days since you last went to the toilet. If it is the latter and you still haven’t been then you need to see and dr and get something to make you go. Constipation can have serious complications if it is going on that long.