When Discovery goes to the over side, they don't have to send a signal right from Terralysium. They can go juuuuuuust to the side of it. Riiiiiiiiight next to it. Then there are two signals in close proximity.
Let's take care of this once and for all. I'll type the exact dialog at the end.
EDIT: I just "love" CBSAA commercials... yadda, yadda, yadda, hurry up... Here we go... getting to the dialogue...
Spock: You detected an anomaly, Captain?
Pike: Number One's running it now but it looks like it's in the Beta Quadrant.
Number One: That's correct, sir... And we have the location. 51,000 light years from the present position.
Number One doesn't say Terralysium. So it can be near Terralysium and still work.
Burnham said earlier she'd send a signal when she gets to the other side. But that doesn't have to be taken literally. They can end up at Terralysium, who knows what happens, and then they can send the signal later, at a different. Even a little later. At a place not too far away from Terralysium itself.