Hello, it could be a hardware issue. Can you reach the iDRAC? Also it can be related BIOS FW version. If the BIOS is too old, recommend updating BIOS and iDRAC together. You almost tried all the troubleshooting steps already. However I suggested to watch this video consider to something overlooked https://www.youtube.com/watch?v=sRwoAuvkFOc&ab_channel=DellEnterpriseSupport

Please try to reset forced if it is not working try to racadm racreset hard command as below 

The Dell RACADM (Remote Access Controller Admin) utility is a command-line tool that allows for remote or local management using the iDRAC. RACADM commands can be run remotely from a management station or locally on the managed system. 
 
In all instances, only the iDRAC will reboot and will not reboot the server (will not affect the running OS).  Please note that there are differences in the commands between iDRAC 7/8, and iDRAC 9.  The commands for resetting the iDRAC include:
 
iDRAC 7/8:
Reset - This will reboot the iDRAC without changing any iDRAC configuration settings

Ex:  racadm racreset   

Reset (forced) - Adding -f to any iDRAC command will force the command
Ex:  racadm racreset -f   
 
Reset iDRAC to factory defaults - this will reset all changes back to factory, REMOVING user and network settings
Ex:  racadm racresetcfg
 
All reset commands can be found in the iDRAC 8 CLI guide, page 105
https://dl.dell.com/topicspdf/idrac8-lifecycle-controller-v2818181_cli-guide_en-us.pdf

Hope that helps!