You need a router for BT Whole Home Mesh wifi. I have installed this and am 100% satisfied with its perfomance which has given me fast, reliable wifi around what is a large house over 3 floors. Installation is a doddle and took under 10 mins.
I had long since replaced my BT router due to poor DHCP handling leading to IP address conflicts. If I were you and were planning to instal the BT mesh, I would bite the bullet and also replace the router at the same time with a non BT one. There are umpteen makes to choose from. Or, you can go down the Google Wifi path which will include a replacement router but is reputedly more difficult to set up.