Extra hubs won’t do anymore to help the weak signal issue than just adding additional repeating devices, either Z wave plus or Z wave plus V2 (the new 700 series). As both @nathancu and @Automated_House suggested, just a regular plug-in pocket socket is usually the easiest, but zwave light switches and zwave lightbulbs are also good repeaters. You just have to strengthen the mesh that you have.
See post 11 in the following thread, then go up to the top and read the whole thing and it should help clear up the confusion about repeaters.
This link goes directly to post 11. The topic title is a clickable link.
As far as single purpose repeater devices, those were needed back in zwave generation three and four, but by Generation five (which uses the marketing name “Z wave plus“), pretty much every device transmits at maximum allowed signal strength anyway, so you get more value out of a dual purpose device like a pocketsocket or light switch. 
I personally like this one from the smartest house Zooz brand. The price is good, you get 2 independently controllable sockets plus a dumb USB charger slot. And it’s a very good Z wave repeater.
Whatever devices you add, remember to do a zwave repair after you have added it or the old devices won’t use the new repeaters for a while. And don’t expect to see much improvement until the next day, it takes a while for the new route changes to propagate through the network.
BTW, You may already know this, but it’s likely that it’s the brick walls which are killing your signal. It’s hard to get signal through those. I’ve known people who had to bounce signal from a second-floor window down to a garage window in order to get it inside the room. And even the wooden garage is are almost always a problem because they tend to have a lot of big metal objects, a lot of cement, even pipes. You just have to do some trial and error to get a good route. Zwave light bulbs can sometimes be useful in situations like these. 