I'm not an AT&T fiber tech but I had the AT&T fiber 300mbps plan installed a few months ago. I get 300+ up and down consistently using the official AT&T speedtest website
http://speedtest.att.com/speedtest/ .
Try running the AT&T official speedtest and see what you get.
Then, based on your pics I would suggest you connect the AT&T gateway/router directly to the cat6 coming out of the fiber/Ethernet converter box (little white box mounted on the wall) for testing purposes. Then connect a laptop to the Ethernet port on the AT&T gateway/router. You could also connect a laptop to wifi and test. This is about as direct as you can get for testing purposes. You should see close to 1TB up and down. If you are not then you need to contact AT&T and complain. If your speeds are good with this test configuration then you have a problem down the line. With all of those cat6 connections there is no telling where the problem is. I would suggest you run cat6 directly from the fiber/ethernet converter to the AT&T gateway/router wherever it is. The installer told me that the latest install procedure they are using uses wifi extenders instead of running cat6 all over the house.
Here is my hardware configuration:
AT&T ground pedestal > underground fiber cable to the outside of the house > fiber going through the outside connecting to fiber/Ethernet converter box mounted on wall in master bedroom > cat6 connected to AT&T gateway/router in master bedroom > wifi to wifi extender in my office/study where my network equipment in located > cat6 from wifi extender to my Netgear router. I run all of my devices off of the Netgear router.
Your Test Results (Tested on 7/18/2019, 7:44:41 PM)
Download Speed: 362.5 Mbps (45312.5 KB/sec transfer rate)
Upload Speed: 374.6 Mbps (46825 KB/sec transfer rate)
Latency: 11 ms
This is how AT&T did our Fiber install. Does it look ok or could it be better?