buy multiple wireless APs is the way to go.
also please note that mobile phone wifi hardware are not much good to start with, even if you get 20-30mbps on the different floor is pretty much good.

using mimo capable router with 3-4 antenna might help improve the speed, if you choose to do single single router setup.

most ideal will be via ethernet port.
but wireless AP as a wifi client is not bad la, can say it is different level compared to what your laptop/semakphone have, so you can still enjoy high internet speed as long as the distance between those two router are not far with wireless connectivity between two routers.

This post has been edited by zerorating: Jun 29 2020, 02:51 PM