@PeteClen 

Hello,

Please confirm below and we can try to fix it:

1. Is the current network layout like below?

Virgin Superhub in modem-only mode--- main P9 (Router mode)---Switch---NAS & Other wired devices

2. What is the LAN IP of the Deco P9 and what is the IP address of the NAS connected to the switch? Can you ping it from a computer that works fine when connected to the switch? Ensure it is obtaining an IP address automatically from the network.

3. How many P9 units do you have in total, is the main P9 the only Deco unit in your current network? What if you install the 2nd P9, connect the NAS to the main P9 with cable and connect a computer to the 2nd P9, will you be able to detect the NAS?

As for the "master and slave P9s intermittently dropping out for a few seconds each time" issue you mentioned, do you mean the Deco network is not stable? Please refer to the troubleshooting guide to narrow down the issue and confirm if the whole system is unstable or just the satellite P9 is disconnecting from the main P9:
Unstable connection with Deco

Good day.