How can I create a Windows Firewall rule to allow only one IP address to connect MS SQL 2017 Express database in Windows 2016 server?
Seems follow-up question of this. However, following are the recommended actions in your case which could be quick release from the cause:


Define static custom port (other than 1433) for the SQL server service


Service restart required here, and the custom port number must be explicitly mentioned in connection string of application. i.e. (DataSource = DatabaseServerIP,PortNumber)

Create new login with sysadmin permissions 
Disable the login sa 


Also, consider applying security benchmark recommendations particularly the sections Surface Area Reduction and Authentication and Authorization. 

Most importantly, do not let the database server expose to public network. Usually, the app server only exposing to public.


  Firewall rule to allow only one IP address (such as 45.23.56.12) 


Start Windows firewall (you can use wf.msc from run command), select entry that related to SQL server from the In-bond Rules list (if not existed create new rule) 

To Allow specific IP
Right click rule → Properties → Scope → from "Remote IP Address" section select These IP Addresses option and Add desired IP Addresses that only needs to be connect SQL Server



To change port numbers
Right click rule → Properties → Protocols and Ports → Enter the custom port (that SQL service listening on - configured in above steps) at Local port section.