No expert on subject but I have Kodi 15.2 on two FireTV boxes and 14.2 on a FireStick. I used ADBFire 1.26 to install the Helix apk. You may want to make sure that your FireTV box is "awake" or out of sleep mode first. You may want to even reset/power off the box and start over.
Next, make sure you have downloaded your Helix from Kodi.TV Make sure to use the ARM version only. Verify your IP Address on your FireTV. Note, your router/gateway address is not the same address of your FireTV box IP Address. (Mine changes, not sure why, but I have a business internet package.)
May want to restart the computer that your ADBFire is on. With all that said, on previous installs this same delay happened to me until I reset everything and started fresh.
Good Luck!