Poor wifi can be caused by several factors, most of which are out of the hands of the ISP ( ie plusnet)
Most ISp routers are for want of a better word, [-Censored-], they will be just up to the job and no more (to cut costs) what router are you using?
Next up the location, and building its being used in, is the router roughly centraly located? how old is the building ( presumably your house) and what is it made from? 60's/70's/80's and upto early to mid 90's buildings will be mainly brick, newer builds will have insulation on the plasterboard that will affect the wifi signal, older stone/heavy brick buildings will also affect the signal, even the best router on the market will not be able to overcome difficult conditions, although the wifi should perform better, and the whole thing should be more stable.
Before you get the ISP ( plusnet) involved try connecting using a wired connection ( ethernet) do you still have problems?
What was your previous ISP? how did their router perform? can you configure this ( old) router to work with plusnet?
tip- if using a new router, instead of entering new SSID/password/connection details into every device, change the SSID and password of your new router to match the old, then you only change one device ( the router) instead of the myriad of devices that connect via wifi in todays world.
