Q: Could it be, that the ddns-service of the rut9x firmware does not support private WAN addresses?
A: RUT9XX ddns-service can support any WAN address. I registered test account on ddnss.de, and configured RUT955 DDnS service (see attached RUT955.png). Then checked ddnss.de and it successfully read my Private IP address (see attached ddnss.de.png). Please, compare your device DDNS settings, you should set them as you see on RUT955.png (change hostnames, username and password from your account).
If issue still persist, please download “Troubleshoot file” from Rut955 (System -> Administration -> Troubleshoot) and send it to me.
Pay attention, because it has Private IP address, we can’t use hostname of ddnss.de to access router. There is one mandatory precondition for DDNS to work - you must have a Static or Dynamic Public IP address. More info, you can find on Teltonika wiki page: