If you want to keep trying to make it work, best of luck to you.
My experience has been that there are many parts of FreePBX that "assume" (that may not be the right word, but its the best I can think of right now), that you're running in Extension Mode and that when you operate in device and user mode, you run the risk that the assumptions made by other parts will turn out to be wrong, with unexpected results.
As a result, I would not use it.
P.S. I have no connection with the developers of FreePBX, and I don't speak for them. I'm telling you what have been my observations over several years. My conclusion is:
DON'T USE DEVICE & USER MODE.