Custom rig imported is glitching out

i believe this is the right category, but if it isnt pls tell me which one

first, i got a oneshot niko model from twitter, with a full rig and everything

the issue is that when i imported it and modifed it a bit, (rigid constraints for holding something, and attachments for it as well) the character just goes around and i think its something with the legs but im not sure on how to fix it. when i import it fresh the legs just go through the ground.

one solution i’ve tried is making parts that stretch across the legs, but it still doesnt work and im pretty new to importing custom rigs and i have no idea what to do lol.

the character you can use to test it is here

if you need any more information just reply with what u need

solved, it was things to do with the hip height and just other things in general