99 times out of 100, when it comes to online gaming issues like this, it turns out to be a routing issue. Like Kneanderthal said, ISPs will almost always tell you nothing is wrong on their end.
I’ve been through this same problem with Spectrum in the past and it took the greater portion of a year for them to fix their routing. My internet would seem fine for almost every other task other than online gaming where I’d get frequent packet loss, ping spikes and jitter.
ISPs are super cheap about wanting to pay premiums for better priority on the backbones. Also, ISPs hate online gaming traffic because they are low bandwidth, but have high packet rates, which eat up resources.