Trouble flashing nrf52840 dongle (Peripheral UART sample)

I am trying to port the BLE Peripheral UART sample to work on the nrf52840 dongle. It's not in the list of supported board but I don't see why it shouldn't work (at least, the BLE advertising part). I created a new app from the sample and created a build configuration for the nrf52840dongle. I then build the code which succeeds without error. Now, I can't use Visual Studio Code to flash the dongle so I followed the steps here: https://academy.nordicsemi.com/flash-instructions-for-nrf52840-dongle/

I tried with both "merged.hex" and "zephyr.hex" but in both cases, when I use "nrf Connect for Mobile" to scan BLE devices, the dongle doesn't show up. I also tried the same procedure but with the GUI programmer but same error. All that happens after the flash succeeds, is that both leds light up (blue and green). I should mention that I successfully flashed a nrf52840dk which works as expected (the BLE device shows up when scanning with Nrf Connect for Mobile).

What am I doing wrong?

Some logs of what I tried:

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~/code/nordic/peripheral_uart/build/peripheral_uart/zephyr$ nrfutil pkg generate --hw-version 52 --sd-req=0x00 --application zephyr.hex --application-version 1 app.zip
|===============================================================|
|## ## ### ######## ## ## #### ## ## ###### |
|## ## ## ## ## ## ## ### ## ## ### ## ## ## |
|## ## ## ## ## ## ## #### ## ## #### ## ## |
|## ## ## ## ## ######## ## ## ## ## ## ## ## ## ####|
|## ## ## ######### ## ## ## #### ## ## #### ## ## |
|## ## ## ## ## ## ## ## ### ## ## ### ## ## |
| ### ### ## ## ## ## ## ## #### ## ## ###### |
|===============================================================|
|You are not providing a signature key, which means the DFU |
|files will not be signed, and are vulnerable to tampering. |
|This is only compatible with a signature-less bootloader and is|
|not suitable for production environments. |
|===============================================================|
Zip created at app.zip
~/code/nordic/peripheral_uart/build/peripheral_uart/zephyr$ nrfutil device program --firmware app.zip --traits nordicDfu
[00:00:10] ###### 100% [2/2 DA8F3E6471E3] Programmed
~/code/nordic/peripheral_uart/build/peripheral_uart/zephyr$ cd ../../ master
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX