wireless networking - How to find a router at an unknown location in a house?

I want to install a WLAN repeater in my father’s holiday house which he rents out to other people.

My father is a digital neanderthal and doesn’t know where his router is, therefore I cannot configure my repeater to this router.

Are there any tools that could help me find that router in the house? I know that there are tools that tell you which Ethernet cable is in use and where it points to, so I figured maybe there are tools that help me find my router?

By “tools,” I don’t necessarely mean software, I’m also thinking about a hardware tool. I tried just going around with my cellphone and searching in the area with the best connection to the network but didn’t find the router.

Edit: Due to some comments that wanted more information about the router: It's a normal ADSL/VDSL router which sends WiFi signals. Distributed by the market leader ISP in my country. It also can do WPS. Here's a link: https://www.swisscom.ch/en/residential/help/device/internet-router/centro-grande.html

Update: Very special. The WIFI Signal was from the router inside my fathers home which is right next to the holiday house. So the Router actually had two different WIFI Signals, with two totally different strengths. In the Holiday house, signal A which my father uses for private stuff had only one line strength on the phone while signal B had 3 to 4. I can't really believe that something like this even exists, but the supporter from the ISP told me that it is like this. I asked "really?" and he said "100%, I can see it in my system".

Answer

If you have a Android smartphone or tablet, you can use the WiFi Analyzer app. It has a screen dedicated to detecting proximity of access points:

Walk around the house and see where the signal is the strongest.