Only about 20% of the UTIs cats have are due to crystals. Are you sure your cat is one of these 20% ? In all the other cases, the diet has no special positive influence.
About 50% of the UTIs are "idiopathic" which means reasons unknown: could be autoimmune, an unidentified virus or whatsoever. In these cases, the diet doesn't help.

What IS important is that the cat drinks enough. Some people here made good experiences with fresh flow fountains. I put a teaspoon of water in my Tigris' food (he is one of these idiopathic patients) and this has improved his peeing greatly.

If it is about diet (my Filou needs kidney diet) I always had to follow the cats. What they don't eat they don't eat I am very glad Filou like his Royal Canin Renal food as he is not allowed to loose weight and he didn't accept all the other diets.

Good luck