I disagree greatly and I actually believe that it is vital this turn. Now that they control all the land in the tundra, Cannala no longer has any reason to push that island -- instead they are likely to push a new island, either the plains or the mountain. Now, radar would provide a fantastic defensive bonus to both of these islands, as it is explicitly stated to work A. on coastal terrain and when there are no obstacles and B. on mountains... which covers the plains and mountains respectively. If we are able to intercept their incoming ships and aircraft, especially when we have torpedoes to hit their ships, we stand a very good chance of rebuffing them from whichever island they choose to attack entirely.
My thoughts on this our RADAR isn't that great at sea provided it works there(the note about a clean sight line suggest we have problems dealing with feedback and would mess with sea use) and scout planes are extremely good at spotting ships unless they run with their lights off at night. In that case(starless night, lights off) they need to use their radios to find each other and keep a good formation and I firmly believe that we can pick up radio signals further then our RADAR towers can see. In the air it would help but, we are getting cheap stingers next turn which will make sure they can't contest our air control. Using the RADAR out in the open sends clear signal to them to fix up their air power lest they be over run. If we act like we got a failed design we can spring a surprise on them next turn when we hopefully get a decent roll.
As a side note on discord the Cannalans said they got a 1 on their design and where quite salty for a while so, while it could be a ruse I'm doubtful. If it's true all the more reason to not tip them off.