Question on Iwatch 4 w/ line

I am new user to iwatch 4 and I live far from any Apple Store so I plan to get my iwatch4 from Docomo but It is been more than one month and no help from Docomo agent. Try to ask them but the replied was "I Don't Know".

Asking about how it going to work if I get it from Apple as it is faster than Docomo and agent replied was " Yes, it is better as it is sim-free".

So how does it work if I can iwatch 4 from Apple store? How can I get a sim for my iwatch??

Can anyone help?