Meshed BL-1300, upstream modem's LAN ports inaccesible to clients

I’ve meshed two GL-B1300s, with the Main Node connected via the WAN port to an Arris Touchstone modem that has several LAN ports with connected devices.

The modem and its devices are on LAN, and the mesh’s clients are on LAN

How do I make the devices connected to the modem’s LAN ports accessible to the mesh’s clients?

EDIT: the devices connected to the modem are slowly beginning to show up. My question now is: how do I enable access to a UPnP media server connected to one of the modem’s ports?

I think you could try setting up port forwarding