Abnormal delay in pinging the gateway ip through WAN 2.5G port.

If you bought it from Amazon US you have contract with Amazon US.

Assuming the device was properly registered, it's registered with Netgear as a US device, in a US location. Device return goes on your expenses, and exchange shipment can go only to an US address, they won't ship it abroad.

Where you live or where you have exported the device to is your own personal choice. 

One additional reason is that you have exported an FCC regulatory device to abroad, FCC regulations (not Netgear!) don't permit to change the radio config to other regulations, so you are operating a device illegal to operate in China mainland.

FMI https://en.wikipedia.org/wiki/List_of_WLAN_channels 

I doubt you can order a US device from Amazon China. Sure, Amazon has (or had) a very poor customer information policy and does not correctly inform their customers, they often allow shipping ways beyond of what is allowed. Yes, Amazon e.g. US does typically not care if the customer is ordering for shipping to an address abroad, so Amazon does (or did) often allow to ship FCC regulatory devices to addresses abroad, handing over the full risk and responsibility for the import to the receiver. It does not matter if Amazon does handle the shipping to abroad: You are the buyer, and become the importer for the destination address. 

By far not a lawyer on trading and importing, however: In brief, this is what applies for self-importing radio equipment into Switzerland (and more generic into Europe). First hurdle will be the lack of CE mark on the FCC devices 8-)

Hello schumaku Nivedita BasuKhodanapur 

The hardware replacement of the device has been completed, however, the problem still exists on the newly replaced device, which means that the cause of the problem can be basically ruled out as hardware.

Previously, based on case number 48171868, the Chinese tech support had escalated the case to the US R&D at my request, and the feedback was that there was a hardware problem, and that's why the device was replaced, which is obviously wrong, so we should correct this mistake, but I'm not sure how to contact the US development team.
At present, the Chinese technical support think they can't reproduce this problem in the lab environment, insisting that it is my network environment, but can't tell me the specific reason, according to the test will be set to WAN port 2 (1G) when this problem can be solved, which means that the WAN port 1 (2.5G) and port 2 (1G) different configurations is the key to solving the problem.
The other difference between the test environments is that they are using the R7000P uplink device, and I am using the R7000.
So can someone please help me to solve this problem completely, thanks.

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?
Be sure your using a good quality LAN cable between the modem and router. CAT6A STP is recommended.