HELP!! P0420

So i had a P0420 code about a week ago, I took it to a shop a week ago and they said i gotta replace the whole cat converter but obviously i didnt do that. then i replaced bank 1 oxygen sensor and cleared the code. (I have the upstream sensor too but only replaced bank 1 downstream) Then i drove it for a week and everything was perfectly fine. I've driven like 400 miles since then and suddenly i was driving and the check emissions comes back on. I check the code again and its p0420 again. Does anyone have any suggestions on what I should do? I guess I'll try to replace the other oxygen sensor but if that falls through I might as well try everything at once. plz help (crying emoji)

edit: actually when i bought the car the code showed up, then i reset it and changed the O2 sensor, then it popped up again. maybe 3rd time it popped up

edit: none of the corner cutting worked. I replaced the Catalytic Converter