SheldonCooper
Yesterday, after chatting with the Samsung support, I tried factory resetting the TV and starting from scratch.
I removed the DigiCard before doing so. During setup, it asked me for the Antenna Type and I selected Cable. It let me input the network details, did the autotuning and found all channels again, as usual.
After completing the setup, I put the DigiCard in, and now, for the first time, it actually started authenticating. It took some minutes to get to 100%, but it worked.
However, the signal issue is still there. Again, it recognizes the channel names, the current programmes, information and durations and everything, but it shows the no signal message as above.
