Unstable RTK fix and low accuracy



I’m using the Drotek GPS RTK kit (XXL base and XL rover).
I have used the boards is completely open areas, with no clouds and i’m having trouble getting a RTK fix. It happens but generally for a few seconds and than oscillates between float and fix.
The accuracy is also quite low in the end. If I take off and land on the exact same position the horizontal and vertical position might be different with distances over a meter.

This is almost the same as what I get without a standard Ublox module.

What can I do to increase accuracy ?