REST API doesn't seem to work
REST API doesn't work with CURL because you don't send cookie after login. By default, Drupal 8 using HTTP Basic Authentication after the login user has a cookie. Browser (include postman) will store this cookie and send to the server any request. So It works normally.

If you want REST API working with CURL. You need to store cookie when request login, and send this cookie to check status login like this:

curl -c ~/cookie.txt
--header "Content-Type: application/json" \
--request POST \
--data '{"name":"admin", "pass":"admin"}'\
https://dfs3w.ply.st/user/login?_format=json


in line 

-c ~/cookie.txt


mean that write cookie to file cookie.txt.

After login successfully, I will read cookie and send cookie to check status login:

curl -b ~/cookie.txt
--header "Content-Type: application/json" \
--header "X-CSRF-Token: -LSxwKsxEnupbeM4NANfV1JLtlGeqK0dRPxhRRSFvkU"\
--request GET \
https://dfs3w.ply.st/user/login_status?_format=json