@dsj 

Are you double NAT?  a WAN IP of 192.168.100.10 indicates its on a private network and unlikely (but not impossible) to be coming from your ISP

If you are behind a router already just set the IP as static to that address?

Another clue is the DHCP leases on the WAN are expiring very quickly which indicates some kind of issue with whatever DHCP server is allocating them to you

  @GRL I use a TC4400 Cable Modem for my ISP which is the 192.168.100.10 IP address. The cable modem does not allow the user to do any network configuration. I'm not sure which exact config you are suggesting would improve the setup.

  @dsj It seems your "Modem" is a router itself and you run Double-NAT (without knowing). Try setting the WAN address of the 605 as static to 192.168.0.10/24, GW 192.168.0.1 and don't get the settings via DHCP. This should stop the fast renewal process and stabilize your connection. 

  @modebm here is the result when I tried using static IP just now.

Again, this product is downright embarassing. To finish the rest of the workday, I have just switched to my DI-604 router from 2003 with 100Mb/s ports. The DI-604 immediately connected to the modem gateway and received an IP using DHCP. I am back online with antiquidated technology that will probably be more stable until I decide what to do next.

  @dsj Did you also set DNS etc? NTP usually is set by FQDN, not by IP. And the WAN up and down was from your setup or instability? Did you also set the default gateway? What does traceroute say?

  @modebm I set everything properly when trying the static option and the ER605 behaved as you saw by toggling the link/connection up and down repeatedly. (just like it did in dynamic)

Considering my ancient router worked immediately with DHCP and the ER605 did not many times throughout today, I am still convinced the ER605 product/firmware is very faulty. All the other issues reported in this community along with the streams of fixes and buggy betas for the product help me feel validated in that the device is the problem.

  @dsj 

Its entirely possible you have a faulty 605, or a faulty cable, or a faulty modem, or its user error.  Why dont you provide a screenshot of your WAN static settings in the WAN config page?  How do we know you arent plugging the cable in and out repeatedly to generate these logs?

Have fun switching to another brand if you want, lots of companies out there have released bad firmware and software for all sorts of products, even the big names.