Cosky, With the R720, as well as all the other 12th generation servers, the minimum RedHat version supported is 5.7. So the issue is probably due to the driver in that version isn't compatible with the new controller. 
Hey community, I am trying to install RH 5.5 on an R720.  It has 8 ( 500GB) drives install, I have them configured as two raid 0's. First raid is two disk for the system, second raid is configured with the other 6 disks for data as raid 0.  I have gone through the raid configuration and the lifecycle configuration.  They are setup to use the BIOS. No matter what configuration I set up the RH install does not see any disks at all. Can I get some insight as to why and how to fix this problem. thanks cosky
Install RedHat v 5.5 on PE R720, disk drives not found during install