Wanted to reply back with this. Even though there does not seem to be much for power management settings in the U2715H, doing a reset energy settings (Menu- Energy- Reset Energy Settings) on all three U2715H resolved the issue 100%. I see this is a common complaint about these U2715H so thought I would post this for future reference.
I have three Dell U2715H monitors running off single DP cable out of Nvidia 1650 card. I never used to have any issues but recently they started not waking up. I can power the monitors off, reboot and still wont wake up. The only fix I have found is getting PC started and unplugging DP cable for about 5 to 10 seconds. Sometimes it takes twice.  I have reformatted the PC recently (no help) and just tried a MSI RTX 3060 card. Same issue. It's quite annoying and I don't think I could get into BIOS if I needed to as I never see that anymore.
U2715H, recently won't wake up