Stylus doesn't work on touchscreen, but finger does

Two days ago the stylus started working intermittently on my Note 3 (SM-N900A) and then, very quickly, not at all. There was no obvious contributing event that could have caused it. The touchscreen still responds to a finger. I understand that there are two separate systems for the touchscreen, one for stylus-sensor and one for finger-sensor. I have tried two different S-pens and I've changed the tip on the old pen, but the touchscreen doesn't respond to any of them. A local repair shop told me that when the stylus no longer works, it's usually a specific cable that needs to be replaced, and that to replace it, you have to break another component. They wanted 100s of dollars to do the work. I'd rather do it myself, but I don't know what parts are needed. Can anybody fill in the gaps?