I kinda got it working. The RUTX as VPN client does still not work. So I set up a site2site with IPSec. That didn’t work at the beginning because the UDM somehow gets confused if there are two IPSec servers configured. In my case one ipsec/l2tp server and one site2site ipsec for the RUTX. So I turned off the UDM IPSec/L2TP server and left only the site2site on. That worked immediately like a charm. If you are interested I can post the confit on both sides.
Now I have just to find a way to get the IPSec/l2tp server working again on the UDM. Seems that both, unify and Teltonika, have their flaws making this simple task almost impossible.