The first thing would be to “ping” the relevant IP address to see if it is responding.  If it is not, the issue is the Internet connection at the IP address or it could be that the IP address of your BAS has changed. This is possible if your BAS is set up to have a dynamic IP address.  If the address responds to “ping”, then you will need to check if there is a port you need to use to access the system.  These would be the first logical steps to take.

Also see the answer to the question: I cannot login remotely to my BAS. What could be the reason for this?