How to connect ONT unit's IP address as a secondary connection?

Hello, I'm a totally new to OpenWRT firmware. I switched from TP-Link firmware to OpenWRT recently. I used to log into my ONT device and to do that at first I added the ONT's IP as a secondary static connection. For that I went to TP-Link's formware then
Network>>WAN>>Primary Connections Type : PPPoE>>Secondary Connection Type : Static, IP: ONT's IP and subnet then save.
Now after switching to OpenWRT I don't have any idea how to connect my ONT's IP as a secondary connection in this firmware. Please help me out.

Likely you are talking about additional wan network (just a subnet with ONT mini web interface isuppose?

Maybe this is what you are looking for?