Hello,
I've been using a BT Hub Disc WiFi extender for a couple years now to Ethernet to my computer easier and have never had any issues, however, I went to use my computer today and the light was flashing red.
I then turned it on and off again and re-connected it to my main hub via Ethernet, this process worked as it should and the light was solid blue, however, upon removing the Ethernet cable, the light went back to flashing red again.
I have tried almost every solution I can find online - those being factory resetting the disc via the small button, resetting both my main router and hub disc numerous times to no prevail. I even tried connecting my hub disc to my main hub via WPS, however, after flashing blue for a while, it went back to flashing red.
Using this knowledge, I have a feeling that there's something wrong with the hardware responsible for the Wi-Fi connection entirely as it doesn't have any issues connecting via Ethernet, this means I'd need to ring the help line and see if a replacement is possible which I don't particularly want to do, I'm posting here to see if there's anything else I've missed that could potentially fix my issue before I try to do that.
Do you have other devices that connect properly with WiFi to the Hub itself? If you do, then that strongly suggests that the disc is faulty - assuming your disc is part of an ongoing service with BT you should have no problem getting a replacement.