Miracast doesn’t use your router, it’s a point to point connection between your device and the receiving device. You should probably get better results away from the router, actually.

However, even with a good connection I think Miracast is pretty bad in practice. Several seconds of delay, desynchronised audio, connection issues, it’s a good concept thst just doesn’t seem to work right on Samsung devices. It’s a real shame, because game streaming services prove thst low latency video connections can be done well, and that’s after passing the signal through a centralised router.