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)