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I was not able to find device on RMS with such serial number, thus it could indicate that other device with the same MAC address is already registered on RMS.
Please navigate to System → Administration → RMS in router's WebUI and make sure you're entering correct serial and MAC address.
If both parameters are correct, please provide the image of router's RMS window.
For security reason, it would be best if you would send such image via private message once you created forum account.