@ZS1YT  Hello, 

Thank you for  your post, may I know do use tp-link AP in Acess Point Mode or other Mode.

For ' I have applied fixed IP addresses to both' may I have a photo of the IP settings on TL-WA901N, if you use default Smart DHCP rather than static IP, will that help? 

1. When TL-WA901N lose Internet, please wire a computer directly to the 25-meter cable and check do you still have Internet from that cable at that time?

2. When issue happens, check what is the IP address on your device that connected to WA901N, and ping router/ default gateway and share with the photos:

   https://www.tp-link.com/support/faq/838/  

Btw, may I know the firmware version / Build number of your AP, and also your network topology, for example router model XX ----- 25 meter cable---WA901N. Thank you.