want to use google home devices. is a mr60 ax1800 mesh router system the wrong choice?

yes i have the mr60. Purchased at costco.

I have the Motorola SB6120 SURFboard DOCSIS 3.0 eXtreme Broadband Cable Modem. Purchased on January 23rd, 2015 on amazon.

Is there some sort of potential problem with a Cable modem? that's very wierd. If i bought something new, would it get better performance in any way?

a couple further details.

We are a 3 person household who use iphone Xs, iphone Xs-max, and iphone XR. have two pc's plugged into ethernet.. 3 google chromecasts, 4 google home mini's, one set of phillips hue smart lights, and 3 smart plugs.

One of the main ways that we are having a problem is that the iphones seem to lose track of what google devices there are in the google home app. We may start streaming netflix from a phone, but then the netflix app on the phone will forget that it is streaming. There is no way to stop the chromecast from that phone, we have to go to a different device on the network to tell the chrome cast to stop casting. Or on the other hand, waiting a while sometimes fixes this problem and the iphone suddenly "remembers" that it was streaming to chromecast. 

This issue was sort of present before i changed routers. but it got worse with the new mr60 router. Now it took me a very long time to set up some of the google home minis. because i needed to set them up through the google home app. and during the setup process through the home app, i'd get a error message saying that it failed to setup and explain to me in rather complicated fashioon that the phone and the device need to be connected to the same network.

I've been in contact with googlehome-support. the guy was super nice and spent a ton of time with me. in the end he concluded that there was something about the router that needed to be changed and he didn't know what. 

I wanted to try disabling the smart-connect feature. so that i could force my iphone to connect to the 2.4 ghz band. I noticed that the smart connect feature on netgear was forcing me to go on the 5ghz band on my iphone xsmax. as far as i can tell with the mr60 there is no way to set it up so that it shows both the 2.4ghz and 5 ghz bands as separate networks which i may choose to connect to. I think that could be the source of the issue that the smart connect feature is trying to handshake with the home mini and put it on the 2.4 ghz band while letting my phone be on the 5ghz band. this in turn makes the google home app setup fail because it "wants" to see both devices on the same band of the network for communication to occur. 

both of the MR60 satellites are plugged in and about 20-30 feet apart on different floors of the house. 

I'm on a 100 mbps connection through mediacom though I usually see 70 mbps through speed test.

Isaac