Plusnet staff support, like the phone service, is still severely restricted. If you would like help from the community, a good starting point is to have a look at This post and supply as much detail from it as possible.
It would also be a good idea to carry out a 'Quiet Line Test' - dial 17070 from a (preferably corded) phone and select option 2 - there should be no discernable noise. If there is, report a phone fault first, as without a clear phone connection, your broadband will not perform correctly.
If you reply to this topic with as much detail as possible, there are plenty of experienced members who can advise on possible remedies.
If you decide to also post on the Twitter/Facebook accounts and get a response from there, please come back here and say so.