Given the long wait times when calling Plusnet I recommend you would follow the online troubleshooter using mobile data to connect to the web site.
Depending on the results of the process a fault ticket will be raised with Plusnet.
I also recommend you but or borrow a landline phone to aid the troubleshooting.
It is useful to prove that you actually have a phone line connection and to test for noise on the line.
A landline phone is not expensive to buy.
For example: https://www.argos.co.uk/product/5529610