I Tunnel is a great one, I second that opinion! It almost has a similar crime/time-travel elements that Signal does but focuses a bit more on characters and the emotions of everything.
I’d also second the Circle recommendation, very tightly plotted mystery drama that has great twists and turns like Signal.
I’ve heard good things but haven’t personally watched Life On Mars and Mother.
Lastly, this isn’t like Signal that much, but you might find City Hunter interesting. It’s a bit more action focused but is centered around a conspiracy and is a great story.