Dell u2719d Monitor not working with Mac Pro 2010

I have Mac Pro 2010 with MacOS High Sierra and with the original “ATI Radeon HD 5870” video card.
I have been using it with old Philips Display connected using DVI-DVI cable.
I recently purchased Dell U2719d and having trouble getting it work.
I have tried connecting the Mac with either Mini-display to Display and with Mini-Display to HDMI cables and get the same behaviour.
When the new Dell monitor is connected and I start the Mac, the display stays black and I get message “No signal from the Display Port”. However if I boot the Mac with my old Philips monitor and then connect the new Dell after login, it works perfectly well and is recognised by the Mac. It continues to work ok even if I then disconnect the old monitor, however if I reboot the Mac with only the Dell monitor, I get again “No signal”.
Any ideas of what is the issue and how to fix it? I hope you can help me.

Try both an NVRAM rest and an SMC reset.

https://support.apple.com/en-us/HT204063
https://support.apple.com/en-us/HT201295

If they don't help, try to start up in Safe mode, https://support.apple.com/en-us/HT201262.

ferrarr said:

Try both an NVRAM rest and an SMC reset.

https://support.apple.com/en-us/HT204063
https://support.apple.com/en-us/HT201295

If they don't help, try to start up in Safe mode, https://support.apple.com/en-us/HT201262.

Click to expand...

Thank you for replay I tried all 3 and still the same results. Any other ideas?

The only other thing I can think of, is that the graphics card, can't power that new ultra hi-def monitor? How much ram does the card have onboard?

@ferrarr It work well, as long as I start with the old monitor and then connect the new one, while logged in.
It is with the original MacPro Video ATI Radeon HD 5870 card with 1G memory.

I would check the monitor's settings - maybe it is set for the wrong connection (or wrong sync, etc) as first choice. Obviously, this is only if it has two connection inputs like DVI and HDMI, for example. I once had this trouble with a Dell monitor, but since I set it to "automatic," it has been working OK.