You can get an optimal solution to this problem by using the Steiner tree solver SCIP-Jack, see https://www.scipopt.org/doc/html/STP_MAIN.php, which is part of the SCIP Optimization Suite, see https://www.scipopt.org/.

You just need to write the problem in the .gr format (see https://pacechallenge.org/2018/steiner-tree/ Appendix A), to be able to solve it with SCIP-Jack

You can get an optimal solution to this problem by using the Steiner tree solver SCIP-Jack, see https://www.scipopt.org/doc/html/STP_MAIN.php, which is part of the SCIP Optimization Suite, see https://www.scipopt.org/.

You just need to write the problem in the .gr format (see https://pacechallenge.org/2018/steiner-tree/ Appendix A), to be able to solve it with SCIP-Jack.