I haven't seen Signal but I had been thinking it reminded me of the american movie FrInequency and when I looked up Signal, wikipedia tells me that it's inspired by that movie. I also read another cnovel, We Live in Nanjing, that was even more similar down to using the same method of communication (ham radio). In the novel I'm reading now they can just hear each other's voices as long as they're in the same room and there's a rainstorm, and as time goes on they're able to see faint outlines of each other too, I predict they will be able to touch each other soon. It feels a little bit more handwaved but with the detail the author puts into the mystery & forensics I'll excuse some handwavey timetravel lol