Since Installing Centurylink Fiber, iCloud fails to connect via ethernet (but WiFi is OK)

I switched my ISP from Xfinity cable to Centurylink fiber about 4 months ago. The speed and value is really excellent. Unfortunately on my main work computer (Mac Pro 5,1, Mac OS 10.14.6, 64GB RAM), I began having issues with sending iCloud email. Anytime I attempted to send an iCloud email from the Mail app I would get a server error. I contacted Apple Support and was put through two long troubleshooting sessions which included all the usual log out/into iCloud, reset of NVRAM, SMC, Safe Boot, and even reinstalling Mojave....to no avail. Meanwhile I could send and receive iCloud email on my iPad, iPhone and MacBook Air while connected to WiFi. I discovered that I COULD send iCloud email from the iCloud web portal, so I began using the workaround of saving a message as draft in Mail, then logging into iCloud webmail to send it. Other mail (Google, and my web domain) work just fine.

Centurylink provides a modem/router model C400XG. I am connected directly to an ethernet port on this device with my Mac Pro, while our household WiFi (a pair of TP Link Deco mesh wifi stations) connects to another ethernet port. The C4000XG's WiFi radio is disabled.

The problem recently got worse, as the iCloud web portal no longer works - I get a "Failed to Verify" when attempting to login, regardless of browser (Safari/Chrome/Firefox). Experimentation revealed that the problem seems to be the ethernet connection. If I disable or unplug the ethernet on the Mac Pro and connect via WiFi, I can send iCloud email and access the iCloud web. I assumed that there was some service or port being blocked by the Centurylink hardware, but did not locate any evidence of that. I've disabled the firewall on the router - made no difference.

It very odd since the TP WiFI network is connected to the same router via ethernet. I have the router providing NAT - it is disabled on the TP devices.

Has anyone experienced this, or do you have advice on how to resolve it? I need the speed and stability of the ethernet connection for my work - and I also rely on several aliases on my iCloud email, so dealing with the workarounds is frustrating.

Thanks in advance!