So likely an issue with the RF Front-End rather than the power too the antenna. You tested the antennas with other equipment, so they are probably Ok.
Depending on the age and how/where they were purchased you might be able to RMA the C099-F9P boards.
I’ve use ZED-F9P on SparkFun and ArduSimple boards, and been happy with both.
I think we’re still a while off before the X20 models appear.