Unstable Speed

[#1]

Problem solved! Using another router (with good WiFi properties) as a bridge from the Plusnet router (which is very basic), I now get good WiFi speeds all over the house.
I did quite a bit of research into the routers and concluded that the plusnet didn't have particularly good WiFi properties whereas the router I still had from TT (also a Sagemcom) had much better WiFi coverage.
Thanks to all for your comments.

[#2]

Hi @AlanN
Glad to hear that this problem has been resolved at your end. Did you use Ethernet cable to connect both routers?
Unfortunately, I chucked my old router last week.

[#3]

Yeah, just a short cable between them-they're side by side anyway. Hope you manage to get yours sorted soon.

Alan.