got a replacement airpod, went downhill

my brother had 2 sets of airpod pros and i lost mine, so he let me have one of his. it only had one airpod though, i didn’t care but i did ask for a replacement airpod. my mom ordered it. before i got it, the case worked and the one airpod inside worked. one thing i also want to know is i could only connect it when the case was on the charger. i thought that was odd. when i got the replacement airpod, nothing worked anymore. the case will only show the green light when the airpods aren’t in th case, nothing will connect nor turn on. how do i fix this?