Ok this seems to be a multi-layered issue.
We have
1. "old" mission with dysfunctional radio due to a new implemented desgin where opening/saving the mission in the me solves it
2. no radio functionality until the engine runs, regardless how we configure the electric system while startup
3. problems to reach appropriate radio stations due to a wrong channel list in the canopy, which reads channels 1 - 20 when it should read 0 - 19, so you actually have to subtract 1 from the shown table to contact the listed station
If you pay attention to these 3 issues combined, contact with ATC should be possible "as normal".
Wouldn't mark this thread as "resolved" though, as the proper corrections in game still need to be implemented.