There are getting to be a lot of variables here. A lot.
If you're going to get good help from internet strangers then much more info is needed.
First off, as others have said already, if the service from the provider is slow or poor then there is no reasonable configuration change that can be made to the wireless setup in the building to improve things.
Second, we're talking about covering 12 condo units with 2 access points. With good quality AP units this is perhaps plausible, but I would wager that it's unlikely to be satisfactory. There are just too many walls and materials to push the wireless signal through for end devices to have a good experience.
How big are the condo units and how many walls are there? What are the materials - drywall and 2x4? Metal studs? Brick walls anywhere?
What WAP models are they? How are they connected back to the router(s)? What wifi channels are being used? Is there a controller software being used or are they each a complete standalone? Are they actually wireless access points or are they each their own router?
One theory I got from the behavior you described of on performing poorly even when right near it - you're double NAT'ing. This would be where you have the ISP line coming in to a router then that connects to another router that is segmenting the networks again. So, the guess here is that the "bad" wireless is the 2nd router in the line and it performs much worse because all its traffic is getting sent up the line to the 1st router and then on out to the ISP. Bad bottleneck. Or it could be misconfigured wifi settings, something about the wireless environment in that particular spot, etc.
Third, is this one line of service being shared amongst multiple units/tenants? Are you sure that is allowed by the provider? Also, one device generating suspicious traffic could result in the entire service being cut for everyone.
What I'm getting at here is that it doesn't sound like you're going to be able to just plug in a wifi mesh system and have your problems solved.
