You've made a common mistake here, in saying that Isildur cut the ring from Sauron at the Battle of Dagorlad. In fact, Dagorlad was the largest battle of the Last Alliance, but afterwards, the forces of Sauron retreated to Barad-Dur, where they held out under seige for seven years before running low on supplies and sallying out against overwhelming odds. Sauron was defeated in the this battle, known as the Great Battle of the Last Alliance, fought on the Slopes of Mount Doom.