Welcome to the boat mate.
In the stock game you do have to ID the target and send the target info to the TDC. Also you can't manually set range past 1300 or so yards.
There are mods that you can use to do both of these things.
TMO 2.0 and above has a ID target button on the orders bar.
TMO 2.5 will let you set range by hand. Other mods will do this also.
You must have SH4 V1.5 for these mods.
Heavy fog is tough and as suggested a constant bearing method is good here, O'Kane or vector analysis work good. One thing about O'Kane is you do not have to be at 90°
The order you input data is important when using the TDC and PK.
Speed first, AoB second and last range/bearing.
Not a long post but a good post.
One thing thou it's best to give your game specs and mods.
Good luck and good hunting
Magic