Yeah, this is a complicated one in such complexes. UPC will be responsible for the connection up to the point where it hits the infrastructure owned by the management company. From that point to within your unit, the management company are responsible for the infrastructure.
When the management company tested, where and how did they test?
As a simple check you can do yourself, unscrew the faceplate of the connection point and check the connections at the back. You'd be surprised how many issues are just down to shoddy connection work.
Are any of your neighbours on Horizon?