So if I'm understanding your question correctly, you want a user to be able to click a "login" link and make a POST request to the "/api/userCheck" endpoint. Depending on the response you want to conditionally navigate to either "/devices" or to "/login" (stay on the same page).

For this you will need to prevent the default "action" of clicking the link from triggering the navigation and call the handleLoginClick callback, then issue an imperative navigation to the target route.

handleLoginClick(e) {
  e.preventDefault();

  const { pass, user } = this.state;

  axios.post(
    "http://localhost:3001/api/userCheck",
    { email: user, password: pass }
  ).then(res => {
      if (res.data.login) {
        this.setState({ fname: res.data.fname });
        this.setState({ isLoggedIn: true });
        localStorage.setItem('loginstatus', true);
        this.props.history.replace("/device");
      } else {
        this.setState({ isLoggedIn: false });
        this.props.history.replace("/login");
      }
    }).catch(err => {
      this.setState({ isLoggedIn: false });
      this.props.history.replace("/login");
    });
}

Ensure the click event is passed to the callback.

<Link
  onClick={this.handleLoginClick} // <-- set handleLoginClick directly as callback
  to="/devices"
  className="btn btn-pink my-3"
>
  <i className="fas fa-lock"/>
  Login
</Link>