Try following this documentation step by step to ensure you didn’t missed a required step. As you’ve not mentioned that might be a firewall issue.
Also I don’t remember exactly how V2 connection string parser works but as documented here Setting up client server connections [SambaPOS] try to match your connection string to that format. For example don’t add ; character after password. Add spaces after ; chars, etc…
data source=[Server Computer Name]\[SQL Express Instance Name]; user id=[User Name]; password=[Password]