I have the z370 Classified K motherboard with dual Killer Ethernet ports.  Been having trouble with internet staying up for about 10 days or so:
> I returned my cable modem & got a new one - same problem. 
> I checked & rechecked the RG6 connections - same problem. 
> I bought a new ASUS RT-AX92U gaming wifi6 router - same problem 
> I bought 2 new Cat6a & 2 new Cat5E cables - same problem
 
My old router was 10 years old & has run flawlessly for the last 8 years.  Being that old, it was time for replacement anyway as it can no longer keep up with my ISP speeds.  
 
I know I could have gotten a dud with the new RT-AX92U.  
 
So, I'm wondering if: while going directly from the cable modem to 1 of the Killer Ethernet ports on the PC, can I plug the other into the wireless router WAN port, so I can verify that either my entire internet is down at the same time, or only my router goes down????  I tried just running the cable out of the 2nd Killer to the WAN port(in) on the router, but nothing happening.
 
Basically the connections would look like:
 Cable Modem > Killer1 Ethernet - Killer2 Ethernet > WAN port on Router for wifi
 
I have a tech coming from my ISP, but won't be here till tomorrow & would like to narrow down the possibilities as much as possible.

Do not confuse a router with a cable modem, 2 different devices for 2 different purposes. Your cable modem supplied by Spectrum is what converts your cable signal to standard ethernet signal for internet connection whether wired or Wi-Fi.  There are quite a few 3rd party cable modems that will not work with Spectrum.   Some of the cable modems from Spectrum also have a router built into them.
 
 
Looking at the cable modem that Spectrum gave me, it does not have a compatibility list, and there should not be one, since the cable modem simply converts your incoming cable into a standard ethernet signal.  Then your router simply routes that standard ethernet signal to various devices for internet access via wired or Wi-Fi connection.
 
 
Right now I have my PC connected directly to the cable modem.  So far today no drop outs, but doesn't really mean anything yet.  I went about 20 hours the other day with no drop outs. 
 
I need a way without adding a switch (another potential point of failure) behind the cable modem, to be able to monitor the internet signal independently between the modem & router.
 
Tried bridging the two Killer Ethernet ports, then sharing each one individually, but got nowhere.

KenMcCI know the difference between Cable modem and a Router.  I was assume (knowing what that stands for)   I have worked with all types of routers and modems for over 18 years,  Many of the devices I have viewed have  router as part of cable interface.  Since I failed to recognize you were not using that kind of device... that was the reason for my post.

 
No worries   
 

KenMcC
 Spectrum has this warning if not using there approved routers. (Probably mostly for lease revenue) >>"You may also choose to buy a modem that has passed minimal testing to confirm that it won't cause harm to our network. Though permitted for use on our network, we recommend against using Minimally Qualified Modems because they haven't been tested to confirm their ability to deliver your purchased internet service speed."  

 
Note that the wording is for Modems not routers...  but they do typically want you to use their router as well so they can charge you a higher monthly fee for their modem/router combo.  It's so much easier to just pay for their modem, but I do prefer to have my own router, & because I had my own router, I was able to get the upgrade from 25Mbps to 200Mbps for no added cost 
 
Tech managed to get out here today because of a couple of cancellations!!!  As soon as he saw the new cable modem he said that was my most likely suspect.   Apparently that brand of modem is known by the techs to have faults.   He went through the whole 9 yards, testing the line coming in from the street to the house, the line from the outside box into the cable modem, the cable modem itself, went out & grabbed a different modem & installed it, & so far so good.  
 
He also complemented me on my RG6 cable building.  He said my connectors were the first ones in a few months that a customer installed that he didn't have to replace lol
 
 

You know that there is a company (I believe is in Minnesota) that actually advertises 198.168.0.1 on the www internet.  Most illegal for the net.  I have never gotten anyone to fix this error.  I can trace it. This can cause your intermittent Dropping network connections.  
Ken