Cereal is kind of by definition no low carb, so it won't be cereal like the conventional stuff. However, there are a number of "n'oatmeal" options out there that are more low-carb friendly. They're generally chia or flax seed based and a mix of nuts and seeds that you can then cook or soak in milk/water overnight (overnight oats).

Cereal isn't the only breakfast item you can do that is grab-and-go, though!

Look into things like bacon and egg baskets/muffins (basically, it's an omelet baked in a muffin tin). They re-heat surprisingly well or can be eaten cold.

Breakfast wraps with low-carb tortillas are another good option. You can make them up ahead of time, then re-heat them when you want to eat.

There's also Bulletproof Coffee and its ilk. Brew coffee, add it and additives like coconut oil and butter (or eggs, or cocoa butter, or protein/collagen powder) to the blender. Blend for a minute, and toss in your to-go cup.

If you don't need to have hot stuff you can just take pepperoni & cheddar sticks. Good in the car while driving. Just be careful not to get greasy fingerprints on the steering wheel/stick shift.

A plain yogurt sweetened with stevia or a few low-carb berries might work. You might look into a lower carb bread/butter. A lower carb cereal with added seeds/nuts. It really depends upon your carb limit. You can always choose to have most of your carb count at breakfast and keep the rest of the day relatively carb-free.