What to do about a rat living in drain line
Your drains likely do not go to sewers, they likely go to storm drains or gutters in the street. So putting Drano is going to put in directly into the environment, where it can harm your kids, your neighbor's kids, your pets and neighbor's pets etc. etc. etc. PLEASE refrain from taking that bad advice...

The rat poison is similar. The reason you don't see the dead rats with that is because it makes them intensely thirsty so they seek out water, then die there. Unfortunately things like raptor birds and other wildlife end up eating the dying or dead rats and get poisoned too.

Get rat traps and bait them with candy bars, preferably ones with peanuts like Snickers bars because they smell stronger. Once you get them all (assuming its a family), get rid of those cheap plastic grates and replace them with metal ones.