The matrix shows availability for:

18/09
LH1121 MAD - FRA 6.10 - 8.45
LH534 FRA - CCS 10.25 - 14.30

25/09
LH535 CCS - FRA 16.50 - 9.25
26/09
LH1122 FRA - MAD 17.05 - 19.40

And lh.com shows the same availability, but only if you look for flights separately. If you try to price the whole itinerary the site shows no availability.

My recommendation would be to find a good TA who can book this for you. There must be a way, but using only online tools might not be enough.