Largely as MrSaffron says, but a few other thoughts that might help.
It does look as though you are on ADSL Max, as that exchange doesn't have ADSL2+ according to samknows
- the site we use to check. (On some aspects that site can be a bit out of date but is probably right for your exchange).
You've no LLU option there either so we need to concentrate on making the best of ADSL Max.
I'll ignore the basic problem at this point, which is the question as to why you are getting the disconnections. That will take a while. Let's concentrate initially on making the best of what you have.
First, as MrSaffron says, you are on the old style DLM
. If you look at the table there, you will see your current rate of 4512kbps should have an IP Profile of 3,500. But you are telling us it is 2,500. That means you disconnected and reconnected either before or after doing the BT test to read the current value. The Member Centre saying 3.5 surprises me in either case, unless it is something like 3.49 and you have rounded up for easier typing.
Which way round did you do things please?
If you read the rest of my page that has that IP Profile page you will see how the system actually works. The IP Profile should rise after a period as described, and the Plusnet copy within a few hours. The way the copy is updated that when the IP Profile changes BT Wholesale should flag it up, and Plusnet checks for the flags three or four times a day. As said on that page, it can go wrong. However the copy never updates immediately the IP Profile does, and it is whichever is the lower at any given time that limits your speed
I have to go now until later today, but it looks as if your router has a Broadcom chipset. That means we can probably do something to speed up the recovery time from the bad periods such as you started the thread with. I need to check out some old methods and discuss the most suitable approach with you after that.
What you must not do
is disconnect and reconnect your router frequently in the hope of speeding up the recovery. You will nearly always have exactly the opposite effect, and delay it. Plus often causing a longer term but recoverable slowdown. Which could be why you never get above 5Mbps now.