@Big_John 

What are the models of the devices you are working with?

What the 500 Mbps tested with your same laptop or was your laptop tested directly connected to the modem? If you have another wired computer try that as well. If you still get 350 Mbps, try swapping the Ethernet cables to see if that improves on both ends.

The other satellite units will lower in speed due to the network, signal loss, etc. The best way to preserve speed is to use the Ethernet backhaul or to wire the units together (I think you are using Decos?): https://www.tp-link.com/us/support/faq/1794/

Go into the Deco app, and in the QoS to see if there may be an option for the speed setting. If there is and the speed is set lower then your subscribed speed, manually adjust it.