I agree your guess that Sonix board is not working. To find it what it is doing really, need to solder wire to monitor UART output. I have not done that.
There is a way to update Sonix firmware via microSD card. Discussed here
One of my SV got from ebay did not connect with WIFI either, fixed by updating firmware.
This readme says
- FIRMWARE_660R_F.bin is the format to use for updates via the
microSD card when the bootloader pin is used on the Sonix board
See linked post to identify bootloader pin. You will need to remove metal shield of Sonix board to access.