I currently work as a software engineer for a company developing network processing and WiFi chips for whole-home gateways (modem + router + wireless access point in the same box) including some Comcast XFinity devices in the US and Liberty Global Horizon devices in Europe, so a surprise is unlikely. 
You can find tons of info on the internet about why range extenders don’t work well and it mostly comes down to sacrificing throughput for range. The main problem for @Maureen1 is that the signal “won’t reach” the guest house. We don’t exactly know what Maureen means, but for an extender to work, the signal does need to reach. It doesn’t mean an extender won’t work at all, though. That’s why I said it’s worth a try because placing the router and extender in near ideal locations might work, and here’s some tips if you want to give it a try.
Start off by making sure your equipment is new. You want a router and extender that supports 802.11ac (sometimes just called “AC”) or the brand new 802.11ax (sometimes called “WiFi 6”). The biggest reason you want one of these is because they support 2 frequency “bands” 2.4GHz and 5GHz. What you want is for the extender and the router to communicate with each other using one of the bands (most likely slower 2.4GHz since it has better range), and as many clients as possible to use the other band. If your main router supports only the older 802.11n standard, the throughput is likely going to be very poor.
For placement, connect your WiFi extender to a 50-100ft extension cord and move the extender around the guest house to try to find the best signal. At the same time, get a 50-100ft extension cord and 50-100ft Ethernet cable (or coax cable) and move the router around in the main house. If that’s not practical, use some common sense by putting them physically closest to each other. The ideal locations would be the router sitting right in front of a window that faces the guest house and vice versa for the extender. Locations near the ceiling and/or on the 2nd story are usually better, too. Another thing to remember is that the best placement for the router to communicate with the extender might not be the best place for the router to communicate with the WiFi devices in your main home.
When you’ve found the best placement, then test your throughput. At a bare minimum, start up two high-def TV streaming clients in the guest house, then try 1 in the guest house and 1 in the main house. But that probably isn’t good enough. Remember that you are sharing the WiFi network in your main home with your guests, so make sure it can support the experience you want to provide for both your guests and yourself at the same time. Also keep in mind that depending on where you live, the signal quality and throughput may change at different times of the day and over time due to interference from neighboring WiFi networks.