Hi,I have the same issue. I have also replaced the Tp-link modem/router. Initially this seemed to fix the issue but then the problem reoccurred.Prior to this issue the AC1600 had been working for 2 months.An example of the problem is my Amazon fire tv stick - working on lan initially, them fire stick says it cant contact internet. If I remove the lan cable the fire stick automatically uses the wifi connection and is able to access the internet.All other wifi connected devices are able to access the internet whilst wired devices are not able to.I am running the latest firmare for AC1600 v2, running default lan/dhcp settings.Any advice appreciated, thanks

Did your PCs get any ip? If there is a yellow question mark, it was impossible for the diagnostic to say a "GOOD".
If you have any pictures to shows your problem, it would be better to know what's wrong.
Plug a cable into LAN port and received no internet access, but wireless worked, which make me think it is a faulty router.

Same issue here: https://community.tp-link.com/en/home/forum/topic/201000