@Dave123
Hello, sorry about that.
1. Please verify what is your network diagram, how many Deco P9 do you have and how are they connected.
2. What if you connect the same PC to your modem/internet directly, will the connection be stable? Please verify the actual speed you can get via speedtest.net. Also, please confirm the wired link speed as the instruction below:
https://www.tp-link.com/support/faq/2265/
3. Please then connect your computer to the main deco and test the speed you can get from it via speedtest.net as well and check the wired link speed.
Note: Please show us a screenshot of the speed test result.
4. Please also run the speedtest on the Deco app and show us the result.
5. When the speed drops to 5M or there is no internet at all, how about the LEDs on all the Deco units, are they red or green?
Additionally, please refer to the below troubleshooting to narrow down the issue:
https://www.tp-link.com/support/faq/2429/
Good days.