How to connect computers via LAN with multiple routers?

Question

System administration

Good afternoon!
The organization has a central CISCO router, it connects to two more routers - asus, and tplink. And each of these routers has its own local network for 3-4 computers. How to combine these local networks into one without touching the integrity of the routers?
Here is the network diagram:

I'm a novice system administrator, so I'm really bad at this. Tell me, knowledgeable people.

Answer 1

Make a DHCP server on the central DLink, switch to DHCP relay mode on the other routers.
All PCs get an IP automatically.
Does the network have DHCP on Windows? If so, then the scheme will be different.
Questions?
PS I hate it when someone pretends to be a girl to get an answer to a question. If I had noticed right away, I would not have answered.

Answer 2

Two simple steps -
That's all.