Santacarl,
I also have E682s in my display and used the FPP Proxy (along with 2 BBBs) to add all of my devices to the Master Pi. Oddly enough, while I can get to either of the BBBs running FPP, when I try to "proxy in" to any of the E682s, I get a partial web page popping up, but I can't change a single item in there. I tried accessing via a tablet and with my home PC and it works the same for both.
A little frustrating because I have to do what you do - disconnect everything, bring in the E682 in it's box, hit the Prog button and go back to default value, access via the web browser (after connecting the E682 to my home network), change any items, set the IP address back to the value it was for the show/display, unplug, and put it back outside and reconnect everything.

Now, while I don't have to change things often, things like this make me want to switch over to Falcon controllers and ditch my E682s. Nothing against the guys that make FPP and included the Proxy, and nothing against San Devices, but I'd like to know why I can't access my controller it through a proxy?