What to add if low-carb muffins don't rise and are crumbly

I followed this recipe for low-carb lemon poppy seed muffins. I only substituted coconut for almond flour and granulated monk fruit for splenda. The mix was really loose and dry when I put it in the tins. I baked it for the max specified 20 minutes and they didn't rise (I didn't swap baking soda for powder, like some do). If I want to make these again, what do I modify to get these muffins to look like actual muffins?

Also, I saw these posts (1 and 2) but I'm doing low-carb and the answers don't fit as I'd expect.