I would like to travel from Frankfurt to Marseille on 04 July at 06:56. In the DB app I can find the following connection: 06:56 Frankfurt - 08:48 Strasbourg - 09:03 Strasbourg - 15:06 Marseille. However, the Interrail app only shows me a connection via Paris, where the train from Paris to Marseille is fully booked. The first train of the connection via Paris is the same as via Strasbourg (06:56 Frankfurt). The next connection to Marseille would not be until 19:00 in Marseille....
Even if I take the train to Paris, there would be a couple of high-speed trains (TGV) that run later but are not displayed in the Interrail app.
So my question is, can I use the connection via Strasbourg even if it is not shown in the app or reserve a seat for this connection? And if so, how do I add it to my trip in the app if the connection is not shown to me in the Interrail app?