We are using Charter Spectrum Cable Internet services with a static IP, and previously it was a dynamic address type account.
Throughout troubleshooting the past year, as I mentioned, we've used several different router models.
Through additional troubleshooting (not my doing, I am a co-technician that is second in "command") we have had 3 different cable modems.
First, it was a SMC D3GN programmed for dynamic IP.
Second, it was a Motorola or Arris of some sort and used both as Dynamic, then upgraded to static IP.
Now, we have a Ubee model (guessing DDW36C) and will confirm next trip along with LAN routing/lack of routing on wired clients, configured with static IP.
Router is a DIR-866L v A1 and firmware 1.0. I see an update IS available, and will try it, but like I said in OP, I had updated along the way similarly with the other models but continued to have "same symptoms".
Wired to the router, we have 2 workstations, and 1 OpenMesh OM2P Gateway that feeds 3 other OM2P in a mesh configuration.
This mesh provides an SSID for public use for our library, and some of the neighbors use it frequently as well. These users are DHCP via the OpenMesh system almost like a VPN would work, not on the routers LAN DHCP configuration.
There is a hidden SSID that DOES get DHCP from the LAN of the router, and again, I can use local resources on the LAN side of the router, but can't route to Internet when this oddity strikes.
We have configured for use, but rarely use the existing wifi built into the router.
We are located in Pellston, MI USA "The Icebox of the Nation"
I had read on other posts changing wifi settings and am currently on Mixed N and G on the 2.4 and auto channel and auto width, WPA and AES on the 2.4GHz.
On the 5 GHz Mixed A, N, AC and Auto channel and auto width, WPA ad AES.
While typing this post, The router actually did this oddity twice in an hour and needed reboot. There were 6 total clients shown as using Internet while it was up, minimal use at that.