Basements are almost always a problem, just like garages, because there tend to be a lot of architectural barriers. Metal, concrete, large metal objects like refrigerators or washing machines, sometimes different kinds of insulation in the walls, water pipes running through the ceiling – – all of it can cause a problem.
The single purpose range extenders are not worth the money: they were useful a few years ago when not all Z wave devices operated at full power, but now pretty much everything does. So you can use most mains powered zwave devices including a light switch or a plug-in pocket socket or even a Z wave light bulb and it will do the same kind of repeating that the single purpose device does. Just make sure its zwave plus to get the most range out of it.
I mention the lightbulbs because those are a popular repeater for the basement because you can often bounce signal down the stairwell using a lightbulb when there isn’t any other place to put a repeater in that area. So that’s just something to keep in mind. Get one that is zwave plus.
https://www.amazon.com/Enbrighten-Wireless-Dimmable-Equivalent-35931/dp/B01KQDIU84/
The other question, of course is if you’ve run out of hops. Where is the hub located?
(The new Wi-Fi mesh hub models do each act as a Z wave repeater, but again they don’t give you anything more than another Z wave repeating device in the same location would. They are more important as far as eliminating Wi-Fi dead spots. )
It sounds like you’ve probably already read the FAQ on whole house coverage, but for anybody else new coming into this thread, I’ll put that here. (This is a clickable link)
Just speaking as a field tech, I’d start out by assuming that it’s hard to get signal into the basement. It’s easy to test that. Take a battery powered Z wave sensor and put it right outside the door to the basement, but on the first floor side, run a zwave repair, and check and make sure it’s working.
Assuming that looks good, now move it to just inside the stairwell to the basement with the door closed, run a zwave repair, and see if you still have communication.
If that’s good, then go down to the base of the stairs and put the sensor there and repeat the process. If you lose the signal at that point, then we got to get a repeater between the top of the stairs and the bottom of the stairs which almost always means a lightbulb, but could be a light switch or maybe a plug-in pocket socket.
( if just closing the door and putting the sensor on the other side of it cuts off communications, you got a different kind of problem and you’ll have to address it like it was an outbuilding.
http://thingsthataresmart.wiki/index.php?title=How_to_Automate_an_Outbuilding )
Once you have a good connection to the base of the stairs, probably all you need is any repeater right in that area, and it should then be able to get signal to most of the rest of the room unless you’ve got a bunch of appliances and stuff.
So sometimes this process is just trial and error, but start out by assuming that the floor is difficult to get signal through and then see what else you can do. 