Do you still want to use the 890L as an AP only or as a main host router?
If you want main host router, then you'll need to have your ISP bridge the ONT service and connect the WAN port on the 890L to the ISP ONT.
If you want the 890L just as an AP, then you'll need to set AP mode on the 890L, then connect LAN to LAN to the ISP ONT. Then look on the ONTs DHCP IP address table list for the IP address it gives the 890L to access it's UI while in AP mode.