Smartthing Mesh router

Hi all, I have a SmartThings Mesh router. When I connect it directly to the modem, it doesn’t establish a network. However, when I connect another router to the modem and then connect the SmartThings Mesh to that router, it works (I can establish my mesh connection). The issue is that this setup creates two Wi-Fi networks in my house and results in lower internet speed on the mesh network.