Hello @Abir2K22 

Regarding the region versions of the routers you purchased, I assume both EU and US versions are available and allowed in the local market, so you may get an EU version or US version due to the availability of the local resellers.

But I don't think this is the reason why you cannot access the device when using the C7 US as the main router, you may check first if you are getting the same WAN IP address and same DNS servers from the ISP modem when using C7 as the main router, you can also try to manually change the DNS servers to some third-parties, such as 8.8.8.8 and 1.1.1.1, then reboot the C7 and check again.

You can also follow the article below to check further settings:

Tether APP fails to manage device remotely?

  @Kevin_Z 

I've been using Google DNS (8.8.8.8/ 8.8.4.4) but the problem didn't resolved.

I've forgot to mention something i.e, when I bought the C7 router online... as soon as I received the product I immediately flashed custom OpenWRT firmware & used the same for 1 week but after not getting satisfactory performance I revert back to default TP-LINK firmware via TFTP .

Was something gone wrong during flashing or I'm thinking that some OpenWRT configuration files are still left in the memory partition causing this issue of TP-LINK cloud access via Tether App. 

Also I'm using the exact same WAN configuration for C7 as I configured in C1200 but still problem persists.

The DNS resolving speed is also degraded after I flashed OpenWRT & revert back to TP-LINK firmware.

According to my experience:  I'm thinking to flash the default factory firmware.bin to the C7 EEPROM via an SOIC8 SOP8 SPI CLIP. 

Will it make a difference? 

Or you suggest something else better ?

Thanks again...

@Abir2K22 

I see. I'm not sure if there could be this difference after flashing the OpenWRT firmware then flashing back, but you could try to reset it to the factory defaults first, then re-install the official firmware to confirm if the issue will be gone. Good luck.