I would like to extend my wi-fi coverage in my house. There are two wireless networks: the first is a private and the second is a guest network. I have an old TP-Link WR841N router, which can be used as an auxiliary router to repeat Wi-Fi signals. The main router is a TP-Link WR1043ND.
A wired connection is always preferred and much better than having to rely upon a wireless backhaul, this makes it much easier for you to set up as well. The basic idea is https://openwrt.org/docs/guide-user/network/wifi/dumbap, combined with multiple VLANs and one AP interface per VLAN.