You're mixing up ADS-B and ADS-C.
CPDLC is only a procedure, you can use different technical means to have it , e.g. over the North atlantic is normally via FANS/1A ( ACARS) if your aircraft is equipped. ( you can also have it via ADS-C or VDL mode 2 , etc..)
Downloading of FMS parameters to ATC is via Mode S ( Radar) , definitively not over the Atlantic.
Do a bit of goolgle to find out how the different systems work.