AusRenovation

You definitely could. I'm always hesitant to use gap fillers outside, just because i don't see them as a long term solution.

But it'll plug the gap for now, and its always easy to replace when it starts looking a bit old and tired in a few years.

You will want to dig out the crack properly first. This is a bit of an art -- take too little or too much away of the weak nearby wall render and it won't look as good. Applying the filler is also a bit of an art if you want its surface to look smooth.

I recommend doing a test run on a small spot somewhere first (that you can dig out again once it has dried). That will help you build confidence and get a working method.

N.B. White filler might be better. I'm not familiar with the product you link to, I think it might be an acrylic based one (water-based cleanup) rather than silicone based, which makes me suspect it's not as hardy but I might be wrong. Test a bit and see if it sets to a nice hard finish (or a weak porous one that stuff might grow in).