When I first installed the m9+ I was getting anywhere between 130 mb/sec in the same room as the modem and 100 mb/sec on the furthest away node. I was using the Motorola SB6121 modem. However since my ISp is giving me 400MB/sec, I decided to upgrade to the Motarola MB7621 modem. 

Now I am getting around 450 mb/sec in the room as the with the modem in it, around 200 mb/sec with the node the next furthest away, and then around 30 mb/sec at the 3rd m9 node (furthest away). And not only that, when I go on the  https://www.speedtest.net/ on my phone it seems to be changing the "servers" as I move around the house even though I have Spectrum cable internet. The coverage seems spotty as in one area I will be getting 200mb and then I take 3 steps away and it drops down to 30 again.

Why would a modem change all of a sudden drop out the coverage of my mesh m9 mesh network? Are there any settings I can change? I'm on the latest firmware.