I see unknown activity in my logs and my admin password is changed few times in past by some one
How can I track and block such activity
You may have a compromised device.
Go to your router manufacturer's support page, download the most recent firmware for it, reset and wipe your router, then install the new firmware.
While you're doing that, access the router only from a single device that you know is clean from malware, then change your network password.
Sorry I can't be of more help, but you haven't given us a lot to work with.