Dual boot Ubuntu 18.04 installation on dell xps 15 9570 RAID or AHCI

I recently bought this a new Dell xps15 9570 computer.
I have partitioned the windows so that there is a partition of about 650 GB empty.
I followed the instructions here
When I run the installation after requesting a wifi connection, the installer says that there is only 4 GB of space left and Ubuntu needs 8 GB.

In my opinion, the software tries to be installed on the USB stick for any reason.
I've tried to disable Secureboot, no help there.

Finally, I tried switching to AHCI (from RAID enabled), which allowed the installer to recognize the partition.
But on these settings, Win will not download (I tried it and it does not find the boot). Before continuing with that. Will it be a problem later? If so, how can I configure the settings so that I can download each operating system I choose?

Thank you
ran