Starting with a summary:  One, signs from Atlas can break when they are dropped or blown off the building by wind. Two, repairs will never be “as good as new”. Three, repairs don’t always work.
If you chip the finish or ding a corner, try a little black marker, carefully applied paint, or whatever. Be creative, you will be surprised at what can disappear once the light colored HDU substrate is not glaring at you.
If your sign is broken and you want to attempt a repair, here is what to use and how to do it:  The only glue that will work is polyurethane adhesive, with the most popular and well-know polyurethane adhesive being the brand name Gorilla Glue. It is important to decide first how you will hold the pieces together so practice with clamps, straps, tape, or as a last resort hand pressure.
Polyurethane adhesive has some properties that effect its use. It is activated by moisture, and it expands into a bubbly froth as it sets. (expansion can be up to 4 times original volume). So, lightly mist both surfaces with water and spread a little glue keeping in mind the serious expansion. Secure the pieces and try not to disturb. You may use water to clean up the plaque face but never any kind of solvent or thinner.
There is a fine line between too much glue and not enough. Not enough will not hold the pieces together and too much will make it a mess. You could use some blue painters tape or some low adhesive tape to protect the face if the glue was to expand. This “might” keep the surface of the sign from getting full of glue.
Last point and most important. The above is purely informational and a suggestion only. See the summary above.