@amscrey what WAN configuration does your ISP Spectrum provide and how is the WAN port on the ER605 currently configured? For a cable modem/ISP, I think it's most common to use DHCP and have the "Dynamic IP" connection type configured in the ER605.

  @dsj 

Thanks for the inquiry/suggestion.

"what WAN configuration does your ISP Spectrum provide and how is the WAN port on the ER605 currently configured? For a cable modem/ISP, I think it's most common to use DHCP and have the "Dynamic IP" connection type configured in the ER605."

Yes, I think you are right about getting IP's from cable ISP's via DHCP using Dynamic IP mode.  I have/had configured both routers mentioned using "Dynamic IP" connection modes available in each routers WAN admin.

  @KJK 

Thanks for the Reply:

"TC8715D is a modem/router combo. I believe its default LAN IP address is 192.168.0.1. Before you connect ER605 to TC8715D, try to change its LAN IP address to an address from a different subnet, say 192.168.1.1."

You are correct - the TC8715D does have router capability and according to the sticker on the bottom it presents its admin on 192.168.0.1.  I think Spectrum charges for providing a router (at least sometimes) and I have never opted to pay for that service.  I think they have the device configured so the routing capability is turned down.  I have not been able to access the admin interface for the device by connecting a laptop directly.  I never see anything rendered when visiting 192.168.0.1.  

I did try changing the LAN IP and subnet used by the TP-Link router - but did not have any luck when trying that.  I tried changing the TP-LInk's IP to 192.168.2.1 thinking I would avoid the confusion/collision of administering TP-LInk ER605 and Modem using the same address (in the case the modem's admin/routing was ever enabled) and also thinking I would use the 192.168.2.xxx subnet to avoid collision with the Linksys 192.168.1.xxx subnet for this temporary period while I still am using that as a router.

The LAN config changes seemed to get applied at first.  The admin interface reset/disconnected (as I was connected via 192.168.0.1) but I was never able to connect to the admin interface on the new address (I expected to connect on 192.168.2.1) and I did not seem to have internet connectivity after making that change.  I think I tried reboot the TP-Link again alone but I don't think I tried reboot both the modem and TP-Link.  Maybe I need to try this again

Thanks to the folks trying to help me figure out what I've got wrong.

Here's an update regarding what I have done since my initial posting.

As mentioned before - I am concerned that I have ONLY been able to establish internet connectivity thru the TP-Link ER605 when also connecting my Linksys router to my cable modem.  When I have tried powering down the modem and the routers and I have only connected the TP-Link ER605 to the modem I have not seen the TP-Link's SYS light come on (or blink) and I have not seen any internet connectivity through the TP-Link in these cases.

Yesterday, after establishing internet connectivity thru the TP-Link ER605 by cycling the modem and both routers and connecting the linksys alongside the tp-link I proceeded to setup the rest of my network as I had previously planned - connecting my switch to the ER605 and hanging a bunch of devices off the switch.  Eventually I disconnected the Linksys and things seemed to continue to work.  I reconfigured the linksys as an access point (using its Bridging mode that turns off the dhcp server) and hooked it up to the switch via one of its LAN ports.

This is the network configuration I desire and it seems to be operating as desired.  My concern remains though.  If I loose power and all these devices go down, based on all the evidence I have so far I expect my ER605 will not establish connectivity thru the modem until I reconfigure my linksys device to be a router again and connect it alongside the TP-Link in order to establish its connectivity.  This would be painful.
 

  @KJK 

"I have never heard about an ISP that would provide customers with modem/router combos and disable the routing capabilities. It's rather the opposite. Some of them make it impossible to disabled the routing capabilities. True or not, I think you need to learn what IP address and gateway you are getting from TC8715D."

Good point.

When I connect and reboot a laptop (configured to get IPv4 using DHCP) directly to the ER605 I see the router/gateway address as 192.168.0.1 w/ subnet mask 255.255.255.0 (as configured in the ER605's LAN settings) and I get assigned an IP assigned to the laptop in that subnet.

When I connect and reboot the same laptop (same config) directly to the TC8715D I don't see a router address - there is a subnet mask listed as 255.255.0.0 .  The IP starts 169.254.   When I issued a 'route get default' on the command line to attempt to see the gateway i got this message: 'route: writing to routing socket: not in table'

  @amscrey if the IP starts with 169.254 that usually means no DHCP server was available to provide an IP. You should also see that the default gateway is blank/missing. You could factory reset the TC8715D to enable the DHCP server again since most devices have it enabled in their default config.