AP Bridge Mode
When you cannot connect two WiFi Router with a cable, you can achieve the using the AP Bridge Mode, as shown in the below diagram the "AP Bridge Mode".
Although both above diagrams are same but for a subtle difference, The "AP bridge Mode" not only connect the Gateway router on WiFi but also allows Clients to connect on WiFi to Second Router (WLAN). In the second diagram the clients of the second router can connect only through cable ie LAN.
When buying the second WiFi router pls ensure it has "AP Bridge Mode", read the manual carefully, because "AP Bridge Mode" can also mean "Bridge with AP" (some call it client)
Recommended Router which has both the “AP Bridge Mode” and “Client” : TL-MR3020
The TL-MR3020 is quite a powerful WiFi router and its pocket size, allows you to carry it along with you.
In the Client Mode the MR3020 connect to the Internet router on WiFi (WLAN) and all clients are connected on the LAN to it
In the AP Bridge Mode the MR3020 connect to the Internet router on WiFi (WLAN) and the clients are connected on the both LAN & WLAN to it.
Example: When the Airtel 4G does provide the speed, we have to move the 4G Hotspot to another location, maybe in your neighbor’s house
If you want both the network on the same subnet, then disable the DHCP on MR3020, The LAN Ip address should be static and IP range within the DHCP Range of Airtel Hotspot, ie if the DHCP on Airtel Hotspot is 192.168.1.0/24 then the IP address of 4g hotspot should be 192.168.1.1/24 and the static LAN Ip of MR3020 be configured 192.168.1.11/24.
How to connect to Airtel 4G Hotspot is detailed in this page. Pls note the MR3020 has switch on the side which must be kept in "AP" as shown below before following the guide.