Hello, you can into system => openMPTCPRouteur
Then "settings wizard" and at the bottom of the page, you have a "add an interface" button. Click on it (keeping eht0 selected), and you can add another WAN interface, based on eth0. You can have up to 8 WAN interfaces.
Then you can go to network => MPTCP, and verify that each WAN interface is enabled for Multipath TCP (and keep one as master)