On the primary router, the one connected to the ISP, find an unused address in its private subnet, which is outside the address range in that router's DHCP address pool.
Set the private IP address of the second router to the address you have found.
Turn off the DHCP service in the second router.
Connect the routers together by Ethernet between a PC/LAN port on each router. The WAN/Internet connections on the second router are not used.
With modern routers an Ethernet cable between the routers can be a normal 'straight' cable; however, with old routers a cross-over cable may be required. You can also use Powerline network adapters rather than using a long Ethernet cable. This will give you a single subnet covering all the remaining Ethernet ports and the WiFi networks from both routers.
Normally, in such a configuration, I use different WiFi network names (SSIDs) so that I know explicitly which WiFi network I am using. If you use the same SSID (and pass phrase) devices may connect to either WiFi network if both are in range.
I hope this helps.