How to use High-Speed Internet Gateway with Existing Router

I have an ASUS mesh system using Merlin and I have many thing configured so I wanted to use router mode and not bridge mode. My main router is downstairs, the mesh node is upstairs and is hardwired to the main one I unplugged the ethernet from the cable modem and plugged in in to the T-Mobile Arcadyan KVD21. The speeds are the same if I use the router or plug directly in to the ethernet on the router. With cox I rec'd speeds of about 160 even though I was paying for gigabit. With TMO 40-60 and the speed varies however the connection appears to be more stable and I Am hopeful that the speeds will continue to increase.I have a ton of stuff connected and there is no buffering. I did lower my MTU which was recommended by others and that helped.  For $25 a month I am going to give ti a change. I am not a gamer