Connect router to another as slave for wired camera

I have tplink stock router connected to fiber internet, and second, old tplink router with openwrt.

If i use join method the wrt router it creates two separate networks:

I need all of them in the same network.

You didn't explicitly state this, but is it correct to assume that you must use a wireless connection between the main router and the second one?

If so, you must use relayd to create a bridge (wireless -> wireless or wireless -> wired).