@gepuk2001
My experience of the Samsung smart TVs is limited to trying to get an off air picture on one while on holiday but my recollection is they start up with a set of choices (Netflix etc.) and you have to find "off air" in the carousel.
Assuming it won't be too painful to do so (re-inputting account information for streaming services) personally I would do a factory reset and then rescan of the DTT channels.
Failing that (and perhaps less painful) there may be an option just to rescan DTT.
I would expect there to be a signal "strength" and "quality" indication on the tuning screen. If they are both green you have enough signal (strength) and the multiplex is able to be decoded (quality). You will only get these indications once you have the TV on a frequency (RF channel) with a "multiplex" on it.
You may be able to step through until you find the first occupied channel or enter numbers to get one.
A postcode search should indicate what the frequencies (channels) used in your area are.
My best guess for the underlying issue is that in some areas there was a bit of a move around to clear space for mobile networks and the programmes are now not where the TV thinks they are.
If the aerial is turned off (as well as the TV) it suggests you have some sort of aerial amplifier and perhaps live somewhere which is difficult for TV reception.
Quite why turning the TV off and on again seems to fix things puzzles me but I think a full factory reset would be worth a try.