Using OrbiAC2200 to boost wifi signal when already using Comcast modem and Asus router

We have Comcast/Xfinity internet, and use the comcast-provided modem with a compatible Asus router we bought on Amazon. The signal in the back of the house is very weak, so we are hoping to use our Orbi to boost the wifi signal to the back of the house. We used the Orbi back when we had AT&T and used it as a router and it worked fine. Now that we have Comcast and have both a modem and a router already in place, I can't figure out how to use the Orbi to help boost the signal to the back of the house. All the posts that I've read have referenced either combined modem/routers or other types of scenarios. Hoping someone in this community can help me set up the Orbi in this scenario to help with weak wifi signal throughout the house. (We bought the Orbi Mini 2 pack AC2200 Tri-Band System from Costco last year.) Thanks!