I accomplished something similar earlier this year, but it was difficult.

I saw availability online separately on SIN-FRA (SQ) and FRA-MUC (LH), but nothing when searching SIN-MUC.

Multiple agents said it couldn’t be done, they couldn’t see the flights, or that the could only book the connecting flight for >24 hours later.

Eventually gave up and went to the ION service center in Singapore (not sure if that’s an option for you) where they were able to manually do it.

I hope you have better luck!