Maybe the directions say how many feet of cable to use?
IIRC: they state a minimum length.
fwiw: a handheld feeding a fixed, mounted antenna at 5w is very competitive with a fixed mount radio at 5 watts.
The main difference is the interface, quality of the receiver and ability for higher power for the fixed mount unit.
HH is nice, convenient, and "at hand". The fixed mount should perform better under poor conditions.
Note that the antenna is a major component for performance. While the HH is handy, that little pointy thing on the HH is no competition to even a 3ft fixed mont antenna, mounted on the hart top. My personal "limit" for a HH is "a mile or so". Can it do better? likely, but since antenna length ( signal capture ), antenna gain ( some is desirable ), and antenna height above water all mean that a fixed mount unit will, in the field, outperform even a quality HH.
Examine your personal use case. My preference is a fixed mount as a primary, with a HH as a convenient backup.
Yes, I know, if you are in a raft, a HH is useful...
As for fixing them, in general there are "no user serviceable parts" in the radio. You might get is running, but it would be a challenge to tune the performance.
Conclusion: Treat the radio as a brick. If it works, good; if not replace. As for antenna, make sure all connections ( there should only be two: at the radio and at the antenna feed ) should be completely corrosion free and mechanically sound., wire should not be ( or have been ) pinched anywhere along it's entire length.
Note these are simply how I handle my radios. Your opinions may differ. But the VHF radio(s) are a basic safety item and deserve careful attention.
( You also have to turn them ON!
)