Upload speed consistently below 1 Mbps. What can I do?

I've been running a speed test (speedtest.net) for the past few weeks. It shows that our upload speed is always less than 1 Mbps. I tried from multiple devices, from multiple jacks around the house and went directly from a laptop to the modem (no wifi router). Also reset the modem several times and had Xfinity reset it remotely several times.

An Xfinity technician visited and said that all looks ok outside the house. He ran a speed test in the house on his own device and also saw the upload is less than 1 Mbps but could not find a reason, so he just left and said they can't do anything about it right now.

This slow speed is causing lots of timeouts and other connectivity problems. I'm trying to work at home but I might be forced to go to an office 20 miles away.

Do you think it might be related to the amplifier in the box outside?  Anything else to try to troubleshoot this?  Obviously, Xfinity is not going to help right now.