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Sorry for the delay in responding. The most likely reason is interference or poor reception.
I would start by checking the coax antenna cable going into the the TV, and making sure its away from other cables, and its not been damaged.
The next thing to do is look at other TV channels to see what their signal strength is showing, select a channel, then press menu on the remote and navigate to Support > Self Diagnosis > Signal Information on the on screen display using the arrow keys on the remote.
The TV channels are broadcast in bands so if you cant see channels BBC 1,2,3, check if you can see BBC 4 (channel 9), or BBC1 HD (channel 101). For Channel 5 see if you have a listing for Channel 4 or E4 (Channel 4 or 28), and see what the signal strength, and error rate is for these channels.
It may simply be the TV turner in your Samsung is more sensitive to interference and hence unable to find the channels, once we know the signal stength of the other channels I can make a recommendation, however can confirm your Wireless modem\router will only help content you watch via the smart hub (over the internet), not the channels you wish to watch via digital terrestrial.
I havent heard from you in a few days, and wanted to understand if I can be of any further assistance?