How configure gateway on a router without WAN

Hi! I’ve a mikrotik router inside a LAN, it’s a CAPSman and it sends the actual LAN broadcast to the WiFi Caps, I’ve added a second SSID with different IP domain, placed it on another bridge with a DHCP server.
Now which way I should use to allow the second IP domain to access the main gateway for the LAN? A simple NAT source of the entire second subnet is enough? I suspect it can work only for a single IP, so I need to implement a masquesrade. How can I do that?
Should it be a masquestade with source nat the second IP domain and output interface the bridge? (wher eis sitting the main LAN).
Thanks

Let’s start off with a network drawing so we’re not trying to figure out what is connected to what.

Hi, thanks for answer, sorry I haven’t created the scheme, I supposed it was easy to describe.
No WAN port, all in the same brigde =LAN1, on the bridge router has an IP and the gateway route is configured too (router can access internet)
I’ve created a second bridge, a new IP address for it, a DHCP server, connected to the slave SSID of Capsman,
now I want the clients access internet on the first LAN, I’ve created a src nat masquerade with source addess the second bridge subnet, and out interface bridge, looks it works, is it correct?