RBK50 dropping connection with new modem Motorolla mb8611

I struggle to see how a standard DOCSIS 3.1 cable modem could cause WiFi interruptions.  It is probably worth a few minutes double checking the Comcast/Xfinity list of approved modems: https://www.xfinity.com/support/articles/list-of-approved-cable-modems 

Motorola's web site seems to think that it is compatible.

https://www.motorola.com/us/mb8611/p 

Directly above is a bit ambiguous.  Generally speaking, there should be some horizontal offset between router and satellite.  There is so much hidden inside walls and ceilings that can degrade WiFi signals that it is often useful to experiment with moving satellites around a bit.  Unless there is a need to access WiFi from outside, little is gained by locating the satellite across the house against an outside wall. Half of the WiFi signal it going "out there" where it may not do anything for you.

I would probably want to look at the Link Rates of those gaming devices and the Link Rate between router and satellites:

• The Orbi app will display the Link Rate of each device.  Often the device itself will report the LInk Rate.
• The maximum user data rates on 2.4G and 5G are shown on the "hidden_info" web page:
  http://orbilogin.net/hidden_info.htm 
  I would expect it to report 400MB/s on the 2.4G connection and 866.7MB/s on the 5G connection.
• For the router-satellite connection, I have to use telnet to access the router.  The RBR50 is compatible with this Python script for enabling telnet: https://github.com/bkerler/netgear_telnet 
  I use a telnet program, such as PuTTY (Windows) to access the router by IP address.
  Enter the regular credentials ("admin" and password).
  Then type satelliteinfo  wifi
• The RBR50 product data sheet says the maximum communication rate between router and satellite is 1,733MB/s.

  That might be possible if they were sitting two feet apart.  Once there is some distance and a few walls/floors between them, it is typical to see a much lower value.  This is what mine show:

root@RBR50:/# satelliteinfo wifi
[
  {
    "mac address" : "A0:04:60:xx:xx:xx",
    "hop" : "1",
    "bridge mac" : "A0:04:xx:xx:xx:xx",
    "backhaul conntype" : "5GHz",
    "backhaul rssi" : "-68",
    "backhaul macaddress" : "A0:04:60:xx:xx:xx",
    "backhaul phytxrate" : "780",
    "backhaul phyrxrate" : "780",
    "backhaul parentmac" : "A0:04:60:xx:xx:xx"
  },
  {
    "mac address" : "14:59:C0:xx:xx:xx",
    "hop" : "1",
    "bridge mac" : "14:59:C0:xx:xx:xx",
    "backhaul conntype" : "5GHz",
    "backhaul rssi" : "-59",
    "backhaul macaddress" : "",
    "backhaul phytxrate" : "866",
    "backhaul phyrxrate" : "650",
    "backhaul parentmac" : "A0:04:60:xx:xx:xx"
  }