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Port forwarding is a different thing to do with firewalls.
The UniFi APs will provide a standalone WiFi network from an ethernet source. You can set the SSID of the AP to be the same as that of an existing network (along with all the other security settings) and you will have some amount of range extension. Alternatively, you can set up a parallel WLAN and choose the stronger of the two.
I bought a Unifi AP (I can find out exactly which one), switched the built in WiFi off and plugged that into one of the ethernet ports on the router. Proper fast and faultless connection everywhere in the house.