I just heard my downlink on the web SDR! The main problem was that the prior owner of the FT817 had set the menu for the front antenna jack, and I am using the SO-239 on the back, bad mistake by me to overlook this. Now all LEDs are green. Thanks to all for the help.
Now, does anyone know how I can mount the LNB and the BamaTech feed on one dish? I originally thought I could insert the feed into the LNB, as in photo A below. But Markus at BamaTech told me this would not work with his duo feed, I would need the feed shown below in photo B (130 euros). I was also told I could not use a dual holder like the one in photo C, because this holder points each feed at a different satellite. It was suggested that the easiest and cheapest way to proceed would be to buy a second dish.