Hi @mirco,
Did you have create a specific forward rule or an exception in the firewall to be able to ping the LAN?
I have a problem similar to yours (please see: [TRB500] OpenVPN connected but not access to LAN).
But on my side, I checked the client and server side routes and everything looks good. On the other hand, when I do a tcpdump on the OpenVPN interface on the Teltonika router, I see pings when I ping the IP address from tunnel network, but I do not see a ping request when I ping to the LAN.
An idea ?