Cant use VPN on LAN but can on WIFI

In your router, do you have an IP address to port forward VPN to? And, have you checked the IP addres you get for an ethernet connection versus a WiFi connection and compare it to what is used in your router?

My assumption is the IP address you are using for VPN port forwarding is assigned to your WiFi connection and not to ethernet.

Edit: I may have confused myself. The VPN IP address should be the server running VPN (e.g. 10.0.1.2). So your router is port mapping the VPN ports to that IP and it wouldn't matter what IP address is assigned to the client. Sorry for any confusion.

Are the Ethernet and Wi-Fi networks on the same subnet?

From the menu bar, select

 ▹ System Preferences ▹ Network

If the Network preference pane is locked, click the lock icon in the lower left corner and enter your password to unlock it. Select the Ethernet connection, then click the Advanced button and select the Hardware tab. Select

Configure: Manually

and

MTU: Custom

Enter an MTU value of 1280.

Click OK and then Apply. Test.

You must apply the change before it takes effect.

If the issue is resolved, experiment with higher MTU values until you find the highest one that works. The higher the MTU, the better the network interface will perform.

If changing the MTU has no effect, restore the Configure setting to Automatically.

What kind of VPN is this?

Delete and recreate the VPN connection with the same settings. Do the same with the Ethernet connection.