Last Thursday, the Vizio tech guy replaced the mobo on the replacement tv. That's the 4th mobo! This time, he also replaced the Wifi board. So far, the replacement tv is now problem-free. Like they say, 4th time lucky? Keeping fingers crossed.
No one's response on this forum provided an answer to the issue. My response may be providing the solution. However, JimY put me on the right track when he advised me to Google it. I did, and discovered that all manufacturers have issues like this, but none were identical to the above. Even the tech guy said it was a first for him too.
An IT friend discussed the intermittent connect/disconnect issue with the Vizio tech in my home, and together we laboriously narrowed the possible culprit to either the tv's ethernet connector being faulty which is on the mobo, or something else requiring the wifi board to be replaced, even though the network is wired, except for my wife's smart phone. (I previously forgot to mention that both tvs' ethernet cabling are double insulated.)
Bottom line: it was very clearly not a modem/router, any other device, cabling, bandwidth, app, file sharing, DNS, DHCP, CEC, firmware, network, or cyber attack issue. It seems to have been a tv hardware quality or quality control issue. Maybe. Anyway, if anything else happens, it would be time to get a refund, and maybe try my luck with - no Verizon, not cable tv - VR or better yet AR glasses?