match default rule :drop

Hello on my zyxel USG20W-VPN router log i have hundreds of these message, coming from all over. They all point to the ipaddress:3389 port of the router.
I use to have RDP enabled, for now i have disabled just to make sure nothing happen. I also have chnaged the router ip address (it is behind) a main router form xx xx xx xx 5 to xx xx xx 55 and i still see all these attempts pointing at ip ...5, even if router is now ... 55
What can i do to stop all these port scan?
thanks