Automatic transmission failure transmissions only a month old

I was driving and it sounded like my front bumper had fallen off and made a clunking noise. Then it would no longer going to any gear. it continued to run but when I tried to put it into gear or put it in to any gear at all it just makes a grinding noise like the gears are trying to catch but can’t. now there’s transmission fluid leaking. I just bought the car two weeks ago and a month before I bought it a new transmission had just been put in. I have the warranty on the new transmission, but I need to know what to troubleshoot before I try to I have them replace the new transmission because of a defect or whatever. I need to be able to prove to them that it’s a transmission that has failed or I need to know what else could have caused the issue. also prior to this failure the D was flashing when you would put it into drive. the guy bought it from said it’s because they didn’t clear the code when they put the new transmission in. which I thought was BS and obviously I was right. but I don’t know much about transmissions so I need some help please.