Did you try connecting monitor on front and back side port of the server?  If you have LCD on the front of the server you can get the iDRAC IP address from that. 
Hello,      I am recently the recipient of a an old server that’s being thrown out at my office. It’s a power edge R720. I plug it in and it fires up like a jet engine. No error codes, lights turn on. However, there is no activity on my monitor - it’s just black.  The monitor is on and plugged in, and both my mouse and keyboard receive power and light up. Any idea what this could be?  It was previously controlled via IDRAC, but I’m unsure how to use this functionality (especially if I get no screen activity). Please let me know if you have any solutions in mind. thank you! 
Black screen upon startup