@terziyski thank you

I had tried with Address Reservation option in DHCP, but sometimes router will release with a random IP. That is why I had added the IP in security level MAC binding.

It's likely that when the same device connects to the Router at another time, the device uses a different MAC address (randomized MAC address) to request an IP address. The Router then assigned a different IP address to that device cause the Router thought of the two different MAC addresses as two devices.