mm43, depending on noise and TX power on VHF the communications range might have been as great as 500 miles for two aircraft at 35000'. For line of sight communications on UHF (higher base path loss) the narrow band FM range is at least 30 miles in my experience. So pure line of sight with AM and 5 watts should be at least 300 miles. At even higher frequencies people have gone mountain top to mountain top with WiFi over 200 miles. So by that check aircraft to aircraft range at 35000' to 35000' should be close to double the aircraft to ground range. Cut off some for multipath as you become tangential to the Earth's curvature and 400 miles should not be difficult.
So a simple "Hey, could you wake up DAKAR for us?" call should have removed ONE obstacle to their being noticed as overdue immediately after they went down. The absence of position reports should have been another cue.