I had a similar issue (constant LAN disconnect messages in addition to no wireless connection detected, as well as frequent reboots and interrupted streaming on things like Netflix with the 25% issue) dealt with this for a couple of years trying various things suggested on here, new cable, router, even swapping out the main board with the Ethernet connector to no avail.
Now that I’ve been running for about a month error free, I’m confident to post how I fixed it. Basically I did all of the above and found the fix to be the uncrimping of the WiFi board cable inside the TV. Even though I don’t use wireless to connect Internet to the TV, it is evident that the signal bouncing of the WiFi board caused by the overly-crimped cable causes the microcontroller on the main board to malfunction over time. Basically follow this: https://youtu.be/gRrBZ2Eu5-I
