How to restrict access to network folders on a D-link modem?

Question

network hardware

I work in a technical school, the task arose to organize access for computers of the computer science class to the network folder of one of the PCs in the public network. Only the D-link DSL-2640U U2 modem with the latest firmware is available. The plan was simple: reassign one of the lan ports to wan, set up routing to the subnet of the general network, and disable the firewall from accessing the PC to all ips except 192.168.1.251. I ran into such a problem that either the firewall opens access to all PCs on a common network, or completely disables it. You may need to configure multiple Vlans. Please tell me the best way to do it. In which direction to dig?