Get no gps fix warning using RTK GPS while flying in offboard mode

Hi guys!

I’m using CUAV RTK for more accurate global positioning.

I subscribe to the /mavros/global_position/global topic to calculate ralative distance between drones. However, I keep getting warning GP: no gps fix and in the same time in the same time distance calculated by global position is far from accurate. I think it’s because No GPS fix warning.
Screenshot from 2020-09-11 14-34-41
When I plug out RTK Base module from my PC and using M8N as GPS, and I don’t get warnings.

Did I subscribed to the wrong topic or any other reason? I don’t know what part could go wrong. Could you guys plz give some hint?

Thanks in advance