Hello @IanS85
That sound to be reasonable distance between PCs.
1. You could try to install the Realtek Chipset Driver here for T3U(Nano/ Plus) and see if that help improve the performance.
https://community.tp-link.com/en/home/forum/topic/509006
2. Disable other WiFi adapter on the computer except TP-link adapter on the same computer
If still the same, 1) may I know was the adapter work fine before ? 2) how often the issue occurs, and what is the error when internet drops out 3) does other device work stable when connected to router WiFi? 4)Can you share the the computer OS and motherboard info here:
How to check driver version of TP-Link adapters and system information of your computer?