Radar doesn't control anything. IF the radar wasn't working (and there are no facts supporting that position) at best it would be a contributing factor. It can't be a causal factor.
There is both a surface search radar and a navigation radar. There are also ESM warning receivers that would have detected a rather close radar from another vessel. And then sonar would also likely have contact. And then there are the lookouts armed with Mk-1 Mod 0 eyeballs, night vision scopes..., etc.
There is no possible way that the OOD on the Fitzgerald could not have know that this merchant was his biggest single problem.