This is a bit old, but it explains about the connection string and setting up different database configurations in detail:
You also need Message Server running on the server machine and in client mode on other systems. If you don’t have Message Server running, changes of entities like tables changing colour won’t happen when done on other terminals. Here is guide on setting that up:
You need to ensure relative ports are allowed for both SQL Server and Message Server on Windows Firewall. You can read about it here:
I know it is quite complex, especially if you haven’t done this before. I just realised the original message I linked to at the start of this topic explains installing SQL Server 2014 and also converting CE to SQL database, but doesn’t go as far as how to configure SambaPOS to connect. So I hope these few additional topics help you in that respect.