Raspberry PI only using 4 GB of 32 GB SD card

I'm currently using a Raspberry PI 4 to run FreePBX. I'm using raspbx to run it. So far everything was going good for a few days. Today I started getting a lot of problems receiving calls. I logged into the freepbx GUI and saw that there was no disk space available. So I ssh'd into my pi and ran df -h. I see that /dev/root was at 100%. I emptied out the database cdr and removed log and temporary files. Still only down to 80%. I now see that there's only 3.5 GB on this /dev/root partition. My SD card is 32 GB. So I ran fdisk -l to see where all of my space went. How do I extend my partition to use that space on my SD card? A screenshot of fdisk -l https://gyazo.com/5fb9e080a03f4b8d6d18fb6f0e075867 A screenshot of df -h