For this method, both monitors need to be using the same type of cable (DP, HDMI, etc.).

1. Power on the PC.
2. Disconnect both cables from the PC.
3. Reconnect the cable that you want to be Monitor 1, in the port where the previous Monitor 1 was.
4. Reconnect the other cable in the other port.

I had the same issue. I don't know if this works with a dock, but it works if connected directly to the graphics card with Windows 10/Nvidia graphics/DisplayPort.