If computers are in same network, even with different ip addresses, they still can see all broadcast and multicast traffic. This means for example dhcp.

If you fully trust your computers, and are sure that no external party can access any of them, you should be fine. But if anyone can gain access to any of your computers, it is trivial to gain access and sniff traffic in all networks.

If you need best security, multiple switches and multiple nics are unfortunately only really secure solution.

Luks FDE, and install dropbear-initramfs, configure ssh authorized_keys and rebuild initramfs. Then you can access initramfs via ssh to type luks password.

Can you access your wan ip when you are somewhere else than on your own lan?

If not, then this is probably just that your router does firewalling and nat is such order that you can access admin interface from local network via wan address.

If yes, then router has some serious misconfiguration.