So far I haven't found a working connection string, it always returns me error like the server/instance couldn't be found, or the remote connection is not enabled.

I double checked in configuration manager as well as SSMS and the remote connection is open and TCP/UDP port 1433 is open.

But I do have a new finding: I create a udl file and double click it to open a connection window, I then input the server/instance name just like the one I put in SSMS (which works fine), then I choose windows authentication, it fails; I tried to create a SQL account and I enabled the mix mode authentication and restarted the server, it still failed. However, I don't see any items in SQL's errorlog

Wouldn't this be weird? What did I miss?

Any clue is appreciated