I don't have this lower grill, I made mine but I did take my lower grill out while my bumper was off. It was easier IMO with the bumper off because you can see where it's actually clipped in and will help avoid damaging anything vs. pulling aggressively from the front although I'm sure others have pulled it off from the front.
The UK version of the Zunsport lower grill installs over the OEM lower grill so you don't actually have to remove it. I'm not sure if the US version will be the same but I would suspect it will install the same way. Check it out for yourself
here. The instructions are in the last pic on their website.