Alizarin, I’m on HRT and it hasn’t stopped me losing weight on 5:2. I don’t know how an ileostomy would affect your diet and calorie needs. It would have a major impact on how your digestive system functions and I imagine it would make it more difficult for your body to absorb all the nutrients in what you eat. I think you would need to consult a dietitian or doctor to have this question answered adequately.

The thing that made the major difference to whether I lost weight or not was what and how much I ate on the “normal” days. It took me a while to realise that what I considered normal was more calories than I needed and it was effectively cancelling out the progress I made on the fasting days.

It’s helpful to work out the TDEE for your goal weight, being conservative with exercise estimate. Then I kept a food diary to make sure I was sticking within this on the normal days. https://thefastdiet.co.uk/how-many-calories-on-a-non-fast-day/ I had to make some adjustments to my expectations of the types of foods I could eat and serving sizes.