linux – Reaching computers on second computers lan though network router

Using Linux.

Main computer ethernet adaptor eth0, ip 192.168.2.5 connected to router, ip 192.168.2.1 router connected to cable modem.

Second computer ethernet adapter#1 eth0, ip 192.168.2.10 connected to router, has access to internet. Second computers ethernet adapter#2 eth1, ip 192.168.3.1 connected to a network hub.

Two raspberry pis connected via usb->ethernet adapter to network hub. ips 192.168.3.20 & 192.168.3.21 everything on 192.168.3.* reaches each other.

I want to reach rpi @ 192.168.3.20 from first computer 192.168.2.5. What do I need to do on computer 1 & 2? I’m fine with using ip’s vs nameserver.