Hammer mode won't help you in the slightest unless you get a huge hammer drill like a Hilti TE-76 and use large hammer drill bits. This still won't help you much as even a hammer drill can bind when hitting the edge of a rock. Actual high quality hammer drills typically have a safety overtorque that is not adjustable. I suspect what you actually have is a drill with hammer drill function, which is different. Compared to an actual hammer drill it gives shorter faster baby hits, and is only really good for drilling 1/4" to 3/8" holes in soft or delicate materials like brick or cinder block or porcelain tile. Any serious hammer drill built in recent years is easy to spot because they have an SDS chuck that only takes SDS bits. You can look up what those are, but suffice to say your augur is probably a regular bit, your chuck a regular chuck, and your drill a regular drill with very limited hammer function. And I looked it up and sure enough that's what you've got.
This application is also significantly more abusive than a drill chuck torque limiter is meant for. You would be better off being careful on the trigger finger and bracing the drill well. I'm not a huge fella and I've taught this to 130lb people, but if you hold the drill like an idiot and hold the trigger when you should let go, a good cordless drill can break or sprain your wrist. If you hold it properly, a 130lb person can completely overpower the drill for the 1/2 second required for a very slow thinking person to release the trigger. If you brace properly you might get a small bruise if you make a huge mistake, but better that than a broken wrist or rapidly wearing out a brand new drill.
Ok how to do it? Put the second handle on your drill securely 90 degrees left of the trigger handle if it has one and you're right handed. Opposite if you're a lefty. Otherwise use your second hand to brace the body of the drill. Make sure your elbows and knees are bent at least a little and put the point of the augur down on the ground. Depending on height, think about which way it will twist the drill if the bit locks up and brace the drill securely on your hip, thigh or ankle. Drilling hip level for example, I put the trigger handle between my legs and the auxiliary handle in front of my left leg. In order to twist, the drill has to push against both of my legs and all the muscle in my arms and core. Sorry drill, I win. Once the hole is started enough to keep the tip straight, continue drilling, but keep the drill spinning (Max speed on high torque/low speed mode is usually about right) and brace strongly against rotation while you slowly apply downward pressure. If you don't apply too much pressure, it will bounce against smaller rocks and knock them loose and allow it to apply more force to a smaller amount of root when it hits. If you do hit something that binds, pull the bit, use a prybar to derock your hole if necessary, and pop the drill into high speed an spin it fast while lowering it very slowly into the hole to tear the roots. Once you're digging through dirt again, pop it back to high torque. Bracing a drill this way is actually very similar to what makes gas powered post hole augers "safe"(They're still pretty risky feelin' and it can be nice to have 3 people on a 2 person drill for a 12" hole unless your people are pretty big, but the principle is the same).
If you're cutting big roots or a significant amount of them, it may pay to sharpen parts of your augur or switch to a hole saw for thick roots. If you hit a hard or big rock with a hole saw and don't pull back immediately it will ruin it very rapidly. If you know how to sharpen one, typically there will be enough damage to only allow one sharpening.
To be clear I'm not ragging on your drill. DeWalt is a decent brand and that particular drill is comparable to my favorite. I was disappointed when I found it took an unacceptable amount of time to put even 1/4" holes in decent concrete, but when working in cinder block and brick it saved me the trouble of packing a second drill. A good SDS hammer drill has a decent chance of shattering tile instead of putting a hole in it, especially drilling near the edge, so I was absolutely delighted to find that the more rapid, softer hits helped it powder delicate materials like tile without shattering them. This saved me the trouble of packing actual tile cutting bits.