high jitter on ISP side

hello everyone, I'm experiencing a ridiculous amount of jitter throughout my ISP route, sometimes I get a 3 seconds delay, this is clearly a congestion issue, right? I've been battling my provider for a while about this and they keep lying to me, pretending nothing is wrong and then sending people that don't even know their names here, when they were telling me that they're sending the technician to visualize the issue personally and contact their infrastructure engineers or whatever after

can this cause them legal trouble for trying to deceive me? I don't have a reliable provider where I live so I have to stick with the current one, it's literally unusable for gaming, and even voice calls get the "megatron" voice effect