Just for your information, I think you probably missed this point :
It created a DHCP server on my usb ethernet adapter. All devices connected to this eth1 interface go through my VPN at ARN. It works like a charm but after some testing it looks like it requires the hotspot_ynh wifi app.
[RÉSOLU] Configuration d'un hotspot filaire - #15 by gauthier67
Did you try installing hotspot_ynh before running GitHub - alexAubin/ynh-extraeth: Configure an extra ethernet interface (typically usb adapter) on an internet cube
I only tested this a while ago on a server that was connected using the vpnclient_ynh app, but it should work without much tweaking…