I also think that an external antenna cannot be connected here. But have you already tried to run the station search outside, e.g. on the balcony (if available) and then try again indoors near the window? Sources of interference such as PCs, microwaves, LED lamps, etc. should also not be close by.
I have this unit and am very happy with it, but I am not in a shielded house. (I know this is of no use to you, but just as an info to others reading this).

Hello gysel
Unfortunately, I don't have an answer.
I had the device replaced twice because I could only tune in one station at a time.
I then returned the device and never used it again.

Greetings
René

Good morning, in my opinion it is not possible to connect an external antenna.... Have a nice day, S.