Wrong: Change IP to the same subnet as your main router. For example, your main router is 192.168.1.1, you change the router connected to the main router to 192.168.1.x.

Correct: Set up bridge mode (AP, Extender, WDS)

It is not very clear to me how you connect. But if this helps:

Wrong: Connect LAN port of Flint to your main router, what ever port.

Correct: Connect WAN port of Flint to your main router’s LAN.