What I did was - I've plugged LAN cable with Internet access to one
router. Configured first Wi-Fi network. Then connected two routers
with LAN cable and configured second Wi-Fi network. But they are still
in one network and every host can access others.
Is it possible to limit this access somehow?
Connecting LAN to LAN put everything on one network.
(a) You can connect the WAN port on each of the two routers to the LAN port on the main network. Let each router have its own subnet and DHCP server.
(b) If you need to, put a small switch or even a small router at the ISP cable to provide the ports you need.
(c) You can try connecting the WAN port on one router to a LAN port on the first router. That may work as well. In (c) the second router might see the devices on the first network but not the other way around.
This prevents access between the two different networks in a simple way.
Someone determined to circumvent this method can do so and it is up to you to assess the risk.
If you want a sturdier separation, you need to implement an added VLAN switch at the main network, and introduce VLANS for each wireless router.
