I have the same filter and it is a bear to get that ring off. I use a rubber hammer and hit the ring to get it moving. Then I move to a regular hammer and hit the metal tabs that stick out from under the ring. There are 4 or 5 of them all around the ring, take a couple taps at all of them. Once you get the ring to move about an inch or two, you should be able to move it by hand. Make sure you are turning the ring the correct direction. It is confusing which way it goes, because you are looking at it from the side, not the top.
Once you get the ring off, open the air bleeder valve and take the top off. The DE grids will just lift out in one piece. Hose that down to get rid of all of the leftover DE.