If you're getting some signal while hooked up to cable, there's probably a problem with one of the coax connectors. Sometimes the crimps are marginal. Sometimes a stray piece of the shield contacts the center conductor. Sometimes connections are loose.
The cable input in the UDC usually goes directly to the signal booster, and from there may go to more than one location. At each location there's typically a splitter involved and a connector could be bad or loose at any of those points.
The good news is that if you test for good quality reception at the signal booster location, there are probably only four connectors involved: one in the UDC where you hook up the outside cable wire, and three at the signal booster. Two are on the back of the booster assembly and the last is the one you connect the TV to at the signal booster location.
If you work on the signal booster, it's a good idea to first pull the fuse in the 12V fusebox. It's easy to short something to the 12V wire while working on the coax.