I bought 2 USB to TTL keys and I also tested with my Macbook pro … don’t you think it’s the Sonoff that prevents the connection? I bought it recently on Aliexpress.
Sonoff basics are sometimes a pain to flash. The method I use is that I connect my FTDI first to the Sonoff, then I hold the button on the Sonoff down for a couple of seconds and KEEP HOLDING IT while attaching the USB side of the FTDI to my computer. This almost always works for me when doing it this way.