Yes, I am testing the signal with a phone, but the WLAN indicator light on the repeater also suggests there is no signal. Additionally, when I set up the repeater, three boxes with configuration info appear on the main setup page: WLAN, LAN and WAN. When I subsequently realise there is no signal and access the setup page, the WLAN box has disappeared from the page.
In terms of a different subnet, I'm not actually sure whether I want this, but I suspect the answer is no. I am a network novice, so I was really just trying to solve the two problems caused by my router/modem being at one end of the house, namely the lack of WiFi signal in the other end of house and the fact that I could not connect my (wired) desktop PC. The reason I have not set the repeater to AP mode is simply because it has always worked in router mode. Since it has stopped functioning, I have tried switching to AP mode, and it seemed to work briefly before also dropping the signal. The downside was that I had to reset the repeater to be able to access the setup page again. I should mention at this point that while the repeater was in router mode, the info boxes referred to above still stated that the WLAN was operating in AP mode.
The router/modem is a Virgin Media Super Hub 3, which is also known as VMDG505 and TG2492LG-VM. The repeater is a generic brand made in China. There is very little else to go on.
The wired connection through the repeater still works fine, and the wireless connection has worked fine for months without issue.