My macbook constantly disconnects from bluetooth network

I have my phone connected to my Mac via Bluetooth. It stays connected just fine. When I try to use the Bluetooth PAN to connect to the internet, that's another story. The Bluetooth PAN works for about 5 seconds, then suddenly disconnects (yet the phone remains tethered via Bluetooth to the Mac). How do I solve this problem?

About my Macbook

OSX 10.11.6

MacBook Pro (15-inch, Late 2008)

2.8 GHz Intel Core 2 Duo

8 GB 1067 MHz DDR3

NVIDIA GeForce 9400M 256 MB

W88440MZ1GN

About my phone

Android 5.1

ZTE Z820