How to restrict access to the network from the network behind the second router?

Question

Computer networks

Hello! Help me understand this situation:
There is a router (A) to which the Internet comes, in its local network there is another router (B) already with its own local network. Because the gateway for router B is router A, then clients of this network can freely go to the local network of router A. The
question is, how to restrict them only to their own network and leave access to the Internet?

Answer 1

If the router knows how, then allocate the port with the router (B) to a separate VLAN and make a separate subnet and NAT for it, then the networks will not intersect and will work as you want, write a firewall on simpler routers, prohibit walking from the IP of the router (B) on the router network (A).