IoTCore: LPWAN gateway not reaching AWS after VPN server

Hi, I have a Multitech Conduit AP Lorawan gateway configured as a Basic Station to our AWS IoT core service. Everything works well so far.

Then we needed to add an openVPN Tunnel for a customer requirement. We have an OpenVPN server in our company, working fine with a lot of other applications.

So I set this tunnel in our gateway, ... and the gateway is no longer visible in AWS.

From the gateway, I can ping: google, remote device management service, and our Service Data Endpoint (in IotCore/settings). I cannot ping my CUPS, but I can't ping it neither from my machine, I supposed that CUPS simply does not have a pingable port.

There is no particular firewall or any filtering on the VPN server.

Could someone help, at least give some ideas?