I have paired a lot of Echo devices, most recently some more new Dots. I used an iPad to do it on since screen is larger than the iPhone.
Make sure no other BT signals are ON in the vicinity, and both devices are ready to go, by turning off receiver for a minute and then back on. Unplug Dot for at least 5 minutes (I had to do this for one troublesome Dot; as was suggested by the Amazon online help).
With both units back on, say to the Dot, "Alexa, pair bluetooth" after the reply, put the receiver into BT pairing mode. then in the Alexa app on iPad, find the Settings for the Dot, and go into the BT section. If no other BT signals exist, look toward the bottom for new devices and you should only see the receiver signal; and select it to pair to. This ought to do it and Dot will tell you it is paired. Dot will tell you how you can easily reconnect to the receiver next time.
Good luck.
FYI, I just now turned on a portable BT speaker I had set up a few weeks ago to work with one of the new Dots. I told the Dot to "connect to bluetooth" and in a moment the two were connected again and working together playing music on the BT speaker. When I am finished listening, I will tell Dot, "disconnect bluetooth" and music will play from Dot speaker, and I can turn off BT speaker completely again. Easy, peasy.