Where exactly is this "Stratsbourg" place???
DB will sometimes route the regional trains to Strasbourg NOT via Kehl but via Wissembourg. It will do you no good to buy Kehl > Strasbourg > Kehl tickets and then to mistakenly travel via the incorrect route! So to get a schedule that takes you via Kehl, enter Strasbourg (only one "t") as your destination, and enter Kehl as a stopover, with 00:00 stopover time on the DB site.
CTS, the Strasbourg transit authority, issues a special ticket in cooperation with the German TGO transit authority for border crossers, the "Europass mini 24h Ticket - Family, which costs €11.20/2 persons. It will get you between Kehl and Strasbourg on regional trains and will also allow you to use public transport all around Strasbourg for 24 hours after the time of purchase:
https://www.cts-strasbourg.eu/en/online-store/fares/tickets/ (click on the "...regional and cross-border tickets" link, then scroll down.)
https://www.cts-strasbourg.eu/export/sites/default/pdf/05BoutiqueEnLigne/Tarifs/tgo_europass-folder_2022-vorab_web.pdf
You can purchase this ticket from a station ticket machine in Appenweier (which is inside the TGO) where you will have a short layover to change trains for the final travel leg to Kehl and Strasbourg. I believe it may be possible to buy it online or perhaps using an app with CTS (I have never tried this however.)
The later you buy this ticket, the longer it is valid - you could perhaps use it the following day as well depending on what time you buy it.
Buy the same ticket in Strasbourg when you are ready to travel back across the border.
