There no such thing as a training period either on FTTP or FTTC. It is full speed as soon as BT and Plusnet set up the account, so it looks like something is wrong.
What does the Plusnet data speed say at https://portal.plus.net/my.html?action=data_transfer_speed
What speed do you get from http://speedtest.btwholesale.com/
Are you testing over Ethernet or wireless? (always test via Ethernet)
What router are you using?
What machine are you testing from? If a Windows PC, is your network adaptor okay in the "Device Manager" and in Network Connections under "adaptor status", what is the speed setting?
Also which version of the OS are you running?
Just a few things to look at from the top of my head and if those are okay, looks like a fault to report.
Ex - Plusnet Customer (2009 - 2023) now with BT