Captive portal to extended via wwan

I would like to extend a wifi network through the wwan interface, but the network requires authentication through a captive portal. When I connect to the network with my router and open a browser the splash screen to authenticate never appears. Is this possible?

Apparently it is ,)

Make sure the wwan and router LAN don't use the same subnet....

The source of the wifi I am trying to extend and my router have different subnets if that is what you mean

Captive portals identify clients by mac address - if you think about it there is nothing else that identifies the client device.
If you use a router to connect to a captive portal then the router presents its mac address to the portal for ALL client devices connecting to the router ( that is what NAT does).

So the first client user connected to the router to log in to the portal will be logging in for the router. ie the router then has Internet access for a full captive portal session.
Any clients connected to the router thereafter will inherit the router's access rights.

That has been my experience. If I spoof the routers mac address and use my laptop to connect to the portal then every device that connects to the router has access.

The issue is I need to re-authenticate and it would be much easier if a splash screen would pop up vs mac spoofing

What you are seeing is a fundamental result of how captive portals work.
If you want everyone who connects to your router to get the splash page then you must bridge ie configure your router as a "wireless access point" instead of a "wireless router".

Are we talking about DNS rebind protection disabling

config 'dnsmasq'
        option 'rebind_protection' '0'

Or via naming

config 'dnsmasq'
        list 'rebind_domain' '/abc/'

I am actually extending a connection that requires authentication through the captive portal. When I connect with a phone or laptop the splash page pops up. When I connect via the router, no splash page to authenticate.

Rebind protection is off on the router

Yes. As I said earlier: