Unfortunately Sigil's only installed on my laptop, so I can't test the hypothesis from this public computer, but (assuming you have your chapter text in a different .xhtml "chapter" than the TOC itself) I think the problem lies in the ID specification. In other words, if I wanted the "#Ch2" tag to point to the "Ch2" anchor name, that would only work *within* the specific .xhtml page I'm working in (i.e., it only points to anchor tags within your TOC "chapter"). If you want to point to a different page outside the TOC, you'll need to specify the name of the file in which the anchor is located. Does that make sense?

I'd need to tinker with Sigil for a bit to figure out exactly how to specify that file path, but it's going to be a few days before final exams are over and I'll have time to play in earnest. If nobody else has answered here with a solution by then, I'll definitely let you know what I found.