There is a depth stop rod on the side of the router that hits a stepped turret on the base of the router. To plunge full depth, the depth stop rod needs to be retracted and locked up as high as you can. The stepped turret should also be rotated to it's lowest step is under the depth stop rod. This should let you plunge the router until the collet touches or is very close to the surface of the wood, but this is not the correct way to set the router depth when you want to plunge rout to a specific depth..
To set a specific depth, you should plunge your router until the bit touches a flat surface, with the router base sitting on the same flat surface and the router NOT running. Then loosen and slide the depth stop rod down until the space between the end of the rod and one of the steps of the depth turret equals the depth below the surface that you would like the bit to cut. There are brass gauge blocks of standard sizes available that you can use to set this gap, but the shank end of drill bits works well for this too. You could also use a scrap of a piece of wood that is the thickness that you want the router to plunge to.
If you wish to make a significantly deep cut, you should plan on using the stepped turret, setting the bottom position for the final depth, and then making each pass until the depth rod hits progressively lower steps for each pass, until you reach your final set depth position. Learn to use this depth stop rod and the stepped turret. It will save you a lot of time and frustration when trying to plunge rout to a specific depths without hitting the collet against your work.
Charley.