Abuse of your internet connection

Hello,
I have just received a mail from the abuse team, regarding some activity on one of my external IP addresses, and mention that if i don’t fix this, they’ll block down my connection.

Among the Explanation problems, it mention that the aforementioned IP address is being used to access the network. After looking up the address, it is indeed the one i use for my NAS server to be accessed (I use to have Plex connected to it, so i can access my media from outside my network, been having this setup for years now). 
What can i do to sort this out? it asks me to close this connection, but if i’ll do, i won’t be able to access my NAS correctly as i want to.