When installing sql server express 2019, there are a few tasks to get it available from the outside world

1) By default TCP/IP isn't enabled, also it uses a dynamic port, and finally even when you set the port to 1433 (Standard Sql Server Port) you still have to use the ,1433 notation to force SSMS to use that port. Details at https://dba.stackexchange.com/questions/62165/i-cant-connect-to-my-servers-sql-database-via-an-ip-address

2) Allow Sql through the firewall - the exe is in "C:\Program Files\Microsoft SQL Server\MSSQL15.SQLEXPRESS\MSSQL\Binn\sqlservr.exe" for SqlServer 2019