My WIFI signal gets to the cottage at the back of the property but not well enough to really use. I am expecting guests soon and would like to boost/extend the signal so they can use it. My router is the Pace 4111N. Can you recommend an extender ?
Depends on how far the cottage is from your house and router. If running Cat 6 or fiber to the cottage isn't an option I would disable WiFi on the Pace and purchase a UniFi AC-LR AP for this purpose. Try to centrally locate the LR and test WiFi coverage at your cottage (keep in mind the LR will need to be wired to the Pace).
If that alone won't reach reliably then add an outdoor UniFi UAP-AC-M to the exterior (preferred) or interior wall closest to the cottage. These APs work off PoE using a Cat 5e or 6 cable so you don't need AC next to them. A wired connection from the Pace to the second AP is always best but if that's not possible just enable the wireless uplink setting on the LR and you'll have your "extender" in the UAP-AC-M but much better than your typical consumer extenders. Unlike a traditional router with an embedded webserver, all WiFi configuration is done through UniFi's free software app using your PC or phone but is really simple.
Good luck.