New Rigol dho804 oscillocope

I use  the power adaptor provided in the accessories box and I connect it to the AC power source (220 V, 50 Hz ).

Well, maybe you should take a multimeter and check the polarity of both power adapters at the output connector.
Of course, the decent instrument should have a polarity protection at the input, but at modern times, nobody can be sure, because the cutting of costs. 

No.  You already have a 15v 3A supply but something isn’t working properly.  You may need a 12v 4a supply but it must be a USB-C PD3 supply as the scope will tell the power supply what it needs.  Don’t just plug any old supply into it: go back to your seller and tell them it doesn’t work

The main question is, what is the power input on his scope. Is it USB-C or a circular bayonet connector ?
Can the OP post a photo of the power input on his scope ?

It’s USB-C

OK, than you need an USB-C power supply.
In this case, I do not understand the photo, which Jan Cumps posted couple of hours ago - https://www.eevblog.com/forum/testgear/rigols-new-dho800-oscilloscope-unbox-teardown/msg4965181/#msg4965181

where at the adapter is clearly indicated as a the circular bayonet connector with + inside and - outside.

But, I am almost sure, that the power source is not a problem. If the scope is able to display a Rigol logo for five minutes and do a restart after that a go to loop, it really looks like a firmware problem.

PavelKCZ  Can I do something to reset firmware before start up operation? To make sure it's not a charger problem.

Yeh, that’s confusing: the referenced PSU has a circular connector according to the datasheet, but the text with that message implies it has USB-C and same poster’s picture of the back of his scope shows a USB-C socket.

I would not recommend any reset or any similar operation.
If the instrument is under the warranty, the only proper way is to contact the vendor and ask for exchange for working instrument or money back.

PavelKCZ You are right , the cost of purchase, shipping and taxes was exorbitant, about $500.

PavelKCZ You are right , the cost of purchase, shipping and taxes was exorbitant, about $500.

Definitely.
You can try to repair an used instrument which you bought on E-Bay for example.
I did this myself, last time it was an Agilent 53132A frequency counter which I bought from China. The instrument comes, but after power up, the panel was dead. I opened it and I found that the flat cable between panel and main board was unconnected. I reconnect the cable and the instrument started without any problem. But it was a used instrument, without the warranty.
In your case, you have a new instrument under warranty, so go and require the exchange or money back. 500 USD is not a small sum. 

dougw Jan Cumps  Andrew J PavelKCZ anniel747 

 I thank you all for taking the time to give me valuable advice and if I find a solution I will share it with you.

That's actually low-cost. It costs a bit more than that in Europe. You could have bought it from an official Rigol distributor from (say) Europe, and got it shipped to you, I suspect for about $600-$650 including shipping, if you were OK to pay import duty on it when it arrived to you.

I know that doesn't help now, but for future test equipment purchases above (say) a couple of hundred $, it might be safer to pay the extra amount to buy from an official distributor. But even with a distributor, you'd still likely have to swallow any shipping charges if things go wrong and you need a warranty repair. 

Yes, this device is under warranty in China for three years, but it does not have a warranty in Egypt.

I must admit that I do not know how the customer protection laws works in Egypt, but I am almost sure that if you will contact the China vendor and ask for exchange or money back, they will arrange for equipment exchange.

The Rigol service just sent me the mail and said that "please try the following solutions:

• Press the power button and quickly press the BACK button on the front panel multiple times. This may reset any incorrect settings.
• Alternatively, place the latest firmware on a FAT32 USB drive, power on the device, and follow the on-screen prompts to upgrade the firmware.
• If these steps do not resolve the issue, please contact our support for further assistance by 'reply all' via this email.' But I don't know what is he meant by back button