Many thanks and much appreciated for your reply. I figured it out the reason why the ram speed was limited, it is due to the previous owner has set the bios -> system profile as custom which he then selected system reliability over performance. I reset that option and the system now recognise the memory speed correctly. Many thanks again for your support.    
Hi all, I have recently procured a used Poweredge R720 with 8x8GB hynix original 1Rx4 pc3-14900 ram installed, everything went fine, the system boot up and I have managed to do whatever I need to do. However, I noticed the ram speed during POST was set as 800mhz and after some research, the dual CPU installed (e5 2643 v2) should support upto 1866mhz. The ram was origianlly installed on A1,2,3,4 and B1,2,3,4, changing to different setup did not change the speed during POST. I have also swap it to 8x16GB samsung 2Rx4 PC3L - 12800 ram but unfortunately that didn't make any different. I wonder anyone can guide me to the right direction please.
Poweredge R720 ram speed during POST