Main router as 192.168.1.254.
I had just to change the base ip of the extender B1300 router to 192.168.1.1 instead of 192.168.8.1 to avoid disconnection and error display on the main router (saying router are not in the same subnet).
(for info…if I go to 192.168.1.254 from wifi of B1300 I see the admin panel of the main router. But from wifi of the main router I cannot see the admin panel of B1300 if I go to 192.168.1.1)