You would need to configure a multicast route.
The camera will have multicast stream settings and you'll need to add the multicast address in use into the router config.
Configuration - Network > IP Routing/Forwarding > Multicast Routes
Set a multicast route towards the source of the multicast traffic, I’ll assume this is Eth 0.
Set the multicast address and mask, and the interface to route traffic to the multicast source then click ‘add’.
You'll need to enable IGMP (multicast) on the interface closest to the device requesting the stream, this will probably be PPP 1 in your case.
This is a CLI option only.
ppp 1 igmp on
Disable the firewall to test, this will check if IGMP is working without potentially being blocked.
It may be simpler to use a UDP or HTTPS stream instead.