I have the Echo UAT, SkyFYX-EXT and a 740 in my RV-6.
I use WiFi connection between the 740 and the Echo. Join the 740 to the Ping-xxxx WiFi network which will be visible when the Echo is on. The WiFi connection between the two has been 100% dependable.
Further, this setup doesn’t tie up the USB port on the 740 so it was available for me to wire the 740 USB out to drive my Dynon EFIS/Autopilot which happily follows whatever route I have in the 740.
Sorry I can’t help with a wired set up but I can say the combo works great together.
As for failing the PAPR, I did first time also. You didn’t mention what transponder you have. I just have an old Mode C that the Echo sniffs to get the squawk code to use for ADSB out. There is a transponder threshold adjustment I had to play with (uAvionics were very helpful with explaining it) and that immediately sorted the PAPR failure out for me. I suspect your ADSB out failure issue is in the settings for the Echo. If you share which parameters you are failing with uAvionics they should be able to help. They certainly did for me.