Usage of OpenMPTCProuter and VPN client on VPS

[mansb123]

Hi, i've the following network architecture: my PC is directly connected to the OpenMPTCProuter which is communicating the VPS. The VPS is an Aruba VPS running Debian 11. I've only one connection between OMR and VPS because for now my scope is to verify if the entire system works. Now, i want to install a VPN client (i'm using ExpressVPN) on the Aruba VPS. When i launch the VPN client, the connection to the Internet stops. What can be the problem
Thanks for answers

[mansb123]

ok i've resolved the problem by modify the OpenMPTCProuter firewall rules. The following figure shows what i've done:

Essentially, i've replace reject with accept in the second line of the table.
The system works but i'm confused about the reason why it works. Enabling the forward option, i think the traffic to wan is forwarded but this action i expect to be done even if the option isn't active.
Can you explain me how to read and interpret the firewall rules table?
Thanks you