Click on the slot and pick the legendary that you want. That will lock it in. A “save and update” button shows up - just click that and it will recalculate.
FWIW, the belt is actually very strong on a fight where there is a lot of tank damage. The current default strategy is based on a Krosus fight, which has constant, heavy tank damage - which is why it ends up being ranked so high. Many very highly ranked resto druids use the belt in mythic+ as well.