Mikrotik RB760iGS with Orbi RBK53

I’m very green to networking and hoping to learn more by getting a router board instead of a commercial router to manage the Orbi RBK53 mesh with wired backhaul.
I have the ISP modem hook to ethr1 of the Mikrotik RB760iGS, Orbi RBR50(AP mode) to ethr2, both the satellites to ether3 and 4 respectively. I manage to get the PPPoE running. The problem being that the RBR50(ethr2) could not locate both the satellites as it should. I believe I’m missing something here, can someone be kind and patient enough to explain the problem. Thanks