I had similar problems with that device.
My problem was that the DVB-C wasn't detected at all.
I played around with it quite a bit, and finally got it working.
I had to change/update the firmware files
*
https://www.linuxtv.org/downloads/firmware/dvb-usb-terratec-h7-drxk.fw
*
https://www.linuxtv.org/downloads/firmware/dvb-usb-terratec-h7-az6007.fw
from here https://www.linuxtv.org/downloads/firmware/#terratec
I think on OpenElec 5.08 it worked without too much trouble.
Later on I replaced OpenElec with plain Raspbian and built my TVHeadend from source, and in the end I went for a VM on ESXi server, with Debian and a self-built TVHeadend install.
I can't remember exactly, but I think the self-built TVHeadend had nothing to do with the DVB-C or DVB-T adapter, but was necessary to get the CI-module working.
If you have a spare SD-card, you could try
* OpenElec 5.08
* Raspbian + building your own TVHeadend
just to see what happens, while keeping your current install intact...