Problem with home Wi-Fi network consisting of 2 routers connected by cable.?

Question

WiFi

The other day I decided to use an old router to increase the coverage of my home Wi-Fi network. For this purpose, I ran a cable (vituha) from the main (Mercusys AC12) to the slave router (Sagemcom F @ st 2804 v. 7 rev. 1, stitched under Rostelecom).
I connected this cable to the LAN1 connector. Changed router IP to 192.168.1.3. DHCP did not disable by setting the address range from 192.168.1.200 to 192.168.1.220. On the main one it costs from 100 to 200. Ip of the main one is 192.168.1.1.
Thus, there are 3 Wi-Fi networks (Mercusys (2.4G), Mercusys5G, and Rostelecom).
And here a problem arose. When connected to a Wi-Fi network, Rostelecom redirects to the web interface of the router, saying that there is no cable connected to the WAN connector. But the main problem is that at least 2 devices are connected to this network and there is an Internet connection. When trying to connect new ones, it redirects to the Web Interface. A similar situation sometimes pops up on Mercusys 2.4G, and it does not transfer to the web interface of the Mercusys router, but to the Rostelecom web interface.
Comrades, I'm just a beginner in all these network technologies, so I ask you, tell me who can.
I really want to get rid of the headache with Wi-Fi networks at home.

Answer 1

First you need to write how the routers are connected. The phrase " I connected this cable to the LAN1 connector. " does not say in which router this was done; and the cable, as you know, has two ends.
The cable must be connected so that the slave router has it in the WAN interface, and the master router has it in the LAN interface (usually in any, they are equal if no special settings have been made).
The slave router must receive settings via DHCP. And he distributes DHCP to his clients. And the easiest way is to have NAT on the slave router.