How to isolate your home network?

Question

Network administration

Good evening fellow connoisseurs, experts and just passers-by.
On the one hand, the question is banal, but something got confused.
There are 3 PCs connected using an old TP-LINK WR740N router, each of the PCs has shared folders with files.

The question is how to make the network and PC "isolated" from foreign devices? So that there is access to the folders of all PCs on my network, but no one else (who connects via wi-fi or lan) gets access to the PC and data. In simple words, deny access to everyone who is not in my workgroup.
There is really no firewall in the router, there is no guest wi-fi either.

I tried to bring the computers to a separate workgroup, but they are still available from the "left PCs" (maybe I configured something wrong.

P.s. now everything is organized at the level of firewall rules (KIS and EIS), denied access to everyone except allowed IPs. But I would like to restrict access at the Windows level, if it is not possible at the router level.
PS2 I will consider all proposals, preferably in more detail, you can link to the article.

Immediately I ask you not to react aggressively, because not everyone was born experts and gurus in their field right away.

Thank you all in advance.

Answer 1

College, but still.
Let's say the router is 192.168.0.1, set the computers addresses manually, approx. 192.168.1.2-1.4, gateway 192.168.0.1, subnet 255.255.252.0
The router also needs to issue a mask 255.255.252.0, but so that it does not issue this mask to clients via DHCP

Answer 2

In Windows 10, there is a creation of a workgroup with a password. If all machines with W10, then connect computers to it - you can only know the password. And then check all open resources and open them only for members of this group.
In general, such things are done through the organization of a VLAN (I don’t know how yours is, but household kinetics already know how), and from the WIFI side, as the creation of a second guest network.