thank you for your prompt reply, I managed to change the port number but 2nd command to restart the service didn't work, I had to reboot the router to make it work.
# /opt/etc/init.d/61AdGuardHome restart
-sh: /opt/etc/init.d/61AdGuardHome: not found
For me, AdGuard is working fine on RT-AC68U, but I want to change the adguard homepage port number, mine is assigned http://192.168.0.1:14711/#, I used AMTM for installation.