Thank you for your answer.
The problem was to get the routers IP at first. Because of an error in DynDNS the router did not announce its IP. To use the SMS command status instead of was impossible too, so I had to visit the installation spot physically.
After gaining contact again I
1. changed “Network/Mobile” connection method from ppp to QMI
2. cleared the SIM-card where 19 of 20 storage places for SMS where in use,
3. switched off storage on SIMS card (SMS administration/storage)
With this modifications SMS commands are accessible again, so the problem seems to be solved but without knowing the real cause.
Up to now I don not understand which SMS commands are stored on SIM an which not.
Looking forward to hear your appraisal