@PieterWiid 

Is the same device connected via WiFi used to connect via RJ45? Possibly the TCP/IP for the LAN port on the device is not setup to obtain the IP via DHCP of the router? Review the TCP/IP networking LAN portion of the device.

If investigating the router, log into the router management interface and review the Security -> Access Control to see if any filtering is in place. Additionally, review the LAN settings and DHCP to see if enough IPs are allocated. I recommend using the System Tools, save a copy of the router settings as a backup before any tinkering.

Take note of the router settings, screenshots or pen/paper will do, perform a reset to default and re-setup to see if this helps before attempt to update firmware. If this doesn't help, update firmware and re-setup router.

Do update the thread and provide which solution works for you.

@Godzilla I had the V1 as primary, V2 as AP wired to V1. Also had Win7 desktop on ether, no wifi adapter, as well as Grandstream Analog Telephone Adapter.

After the ethernet on the V1 failed, I reconfigured the V2 as primary router, connected only the PC, same problem.

I tested using a Linux laptop, explicitly disabling the wifi. I was able to connect to the management app using ether, but no routing to the WAN

@PieterWiid 

I recommend downloading the User Guide and follow the manual to review the LAN setup, along with DHCP and Internet. Possibly review the Security portion, where the MAC filtering is in place. However, having access to the management interface allows you to perform a re-setup, that might be a possibility too.

https://www.tp-link.com/us/support/download/tl-mr3420/v2/ (English)

https://www.tp-link.com/de/support/download/tl-mr3420/v2/ (German)

@Godzilla Been there, done that. Note -- this router's been working for AT LEAST 5 years. The ethernet ports stopped working while my wife was surfing the net, in the middle of the day. There is NO security setup on the ether ports. To repeat wifi works, ethernet does not route to the WAN, but DOES connect to the management app.

@PieterWiid 

Is there another PC available to test, by connecting to the LAN port(s)? Next, review the TCP/IP network setting for the LAN/RJ45 interface of the PC with issue, possibly DHCP turn off somehow or static IP was entered without proper gateway? Hence, PC cannot get out of the LAN.

@Godzilla I used a Win7 desktop and a TP61 with Linux Ubuntu 14 LTS. Windows refuses to talk AT ALL, because it can't see the internet.

@Godzilla If DHCP were off, then I would not be able to access the internet using Wifi. There are not separate DHCP configs for LAN and Wifi