Since I started using zerotier in CoreELEC I stopped using OpenVPN on my router because it is much easier to configure when you understand how it works, you do not need DDNS and you do not need to publish the IP or open ports or support vpn attacks from the internet.
For your case my advice is the following:
1.- Check the OpenVPN configuration of your router and export the access configuration ovpn file.
2.- Check the exported ovpn file with a plain word processor and change whatever is necessary (In my case, as my router didn’t even know the DDNS name, I needed to change this, my vpn port was not the usual one either).
3.- With a smartphone install the OpenVPN client application and load the ovpn file from the previous step. Keep in mind that the OpenVPN connection will not work from within the home network where the router server is, you will need to disconnect the WiFi of your smartphone for it to work. When this works you can move on to the next step.
4.- When the above steps are done, then you can try Zomboided VPN Plugin in CoreELEC. My advice is to delete the addon completely and start from scratch, use the ovpn file if possible, and don’t do weird things like change the permissions of any files (sorry, I don’t know the Zomboided VPN Plugin).
Remember that an OpenVPN client will not work within a home network when the server is the router.