If you are going to miss a connection then talk to staff, on train or at the station, the earlier the better.
If it is a booked compulsory reservation train then it is up to them to get you on a later train at no cost to you.
If you're about to miss a train, speak to DB staff and see what they offer. Worst case : at the train station or before boarding the train on the platform. But if possible do it earlier.
They will issue you a new reservation free of charge on the next available train (whether in 1st or 2nd class).
However as trains between the two countries are very popular you may have to wait a long time or take an alternative route (regional train via Appenweier for example).
The best option would be to have a margin for this important connection as DB isn't known for its reliability...