I have a j-pouch, since I was 14 due to UC which was supposed to be the solution to all my problems. Was rushed into surgery without even knowing what a colostomy bag was (I was less than 6 stone, and people were convinced I just had an eating disorder, wtf) and as you can imagine was horrified when I woke up to see this protruding from my stomach, and plus the immense pain I was in. Turns out after all the surgeries and having the J-Pouch created it was Crohns that I had so things continued to get worse, I'll spare you the details but I couldn't go back to a colostomy bag because of complications. I'm currently stuck between a rock: an alleged failing J-Pouch, and a hard place: no other options aside from colostomy bag.
I was advised the same as yourself, and stuck to a high carb - low fiber diet for years which left me unable to do a lot of the stuff my peers did. Some weeks were okay, some weeks not and it was hard. It's still hard. I've been doing keto on and off since October 2015 and I find that it helps a lot with the pouch. I don't eat a lot of fiber on this diet, I just focus on my fat, protein and staying away from carbs. Things are okay when i do keto, I don't know if it's because of the amount of carbs/food I eat now compared to before, or if the IF helps too because I don't eat until lunch time so I wont have to be running to the toilet all morning.

Currently some of the food I eat: baby-bel cheese, ham, salads (lettuce, cucumber, sundried tomatoes, beetroot), all kinds of meet, cream, butter, eggs, pistachios, cooked broccoli and cauliflower, spinach etc. I generally find that raw fruit and veg such as carrots, apples, nuts, seeds and high fiber foods stress out my pouch so it's easy to stay away from them. Medication has helped with the pouch too to some extent. If you have any questions feel free to PM me and I'll talk you through what has helped me.