You will not be able to manually enter a digital channel (anything with a 2.1, 4.2, etc) as those channels must be "discovered" by TV's tuner to determine what the actual broadcast frequency is. Channel 2.1 in Miami is not going to be the same as channel 2.1 in Detroit.
There is a chance that if your TV has the manual add option you might be able to add an analog channel (3.0, etc) and it's likely the DVD is on 3.0 or 4.0.
The part I am most confused about is where you are doing the scan. I assume that the sound system you describe is NOT involved with this scan but only the TV. If so, just select TV on the radio and do a scan. Then try to add the analog channel that is used by the DVD. If this works you should never have to scan again when switching between inputs. If you can't add the analog channel see if you can find the Additional Scan option (on/off or check uncheck, etc.), switch to DVD and do the additional scan. As soon as you see 1 additional analog channel you can exit and it will be added to the list.