It would be helpful if you provided the topology of your network. What router are you using?
I assume, that the router acts as a server. Unless you have load balancing configured, the router should only use only one WAN interface to forward traffic to/from internet. Thus, with every additional WAN interface, I would suggest to check it's metric in a routing table or priority in Network -> Interfaces section in the WebUI to see, which interface is used to access the internet.
Since each WAN interface has a different public IP, this has to be reflected in the client's .ovpn configuration file. The client must know the IP of the interface of the server. Could you check this?
If the above do not help to solve your issues, please share your VPN configurations.