Massive upload speed reduction

[#1]

Since upgrading to Fibre back in the summer, I've had a pretty rock steady upload speed of around 7-8mbps. However, sometime between the 18th and 21st of November, that speed dropped dramatically to around 1.8mbps and has remained there ever since.
My line is still currently syncing at around 7mbps and download speeds are unaffected.
To try and solve this problem I've tried numerous things to try and pinpoint the issue. I've unplugged everything else from the network leaving just my PC, I've run speed tests from my phone whilst the PC is off - but all tests still return an upload speed of around 1.8mbps.
Naturally thinking there is something strange afoot, I've had a ticket running with Plusnet for over three weeks. The suggestions have ranged from BT regrading my line to accommodate more customers to suggesting I was uploading massive amounts of data which was skewing the speed tests. They have now settled on saying that BT have kicked in DLM and that there is nothing they can do about it.
To say I'm miffed at this news is saying it lightly! Apparently there is no 'minimum' upload speed guaranteed and so I must take it on the chin and accept this dramatic loss of speed.
It's very frustrating that my fairly average speed (with regards to what people can get in other areas) to begin with has now been hamstrung to these levels. I can understand if it had dropped a meg or two, but to be quartered leaves a very sour taste

[#2]

you wouldn't be using a Fritzbox 7390 would you

[#3]

Nope, it's an Asus DSL-AC68U AC1900. I had the same results whilst using a TP-Link TD-W9980 too.

[#4]

Please could you confirm if you've tested the speeds using a PPPoE connection?

[#5]

Hi Harry. I have tried to set up a PPPoE connection as shown on the guide in the support section. But I get an error code of 651 when setting it up (between steps 7 and 8 in the guide after hitting the connect button) and then when I plug in the Ethernet cable and try to connect, I get error 720.

[#6]

When trying to set up the PPPoE connection, are you doing this directly through the openreach modem?

[#7]

I am, yes. I noticed the guide is for Vista/Windows 7 but I presume it's all the same for Windows 8.1? I completely unplugged my router, followed the instructions to set up a new connection then after clicking connect, it says error 651 but do I want to continue with the connection anyway, so click yes. Then I plug the Ethernet into the Openreach modem, click to connect but error 720 prevents the connection being made. The DSL light was lit solid green on the modem.
Thanks for your help btw I do appreciate you taking the time

[#8]

FWIW  1.8M upload is what you get when you are capped at 2M on the 40/2 product.

[#9]

except he isn't as shown by the opening post

[#10]

Ok, before trying to set up the PPPoE connection, could you log in to the router and manually disconnect the connection to the router.
Please let me know if that give any different results.

[#11]

Ok, I've just tried that, but still get the same result Error 651 seems to be whilst the Ethernet cable is unplugged, 720 is when it is plugged in. The attached is the full 720 error message.
Thanks again everyone for all the replies!

[#12]

Are you entering the login name as yourusername@plusdsl.net?

[#13]

I am yes!

[#14]

There aren't likely to be any minimum guaranteed speeds, but the BTWholesale minimum planned upstream throughput speeds are 2Mbit/s for 80/20 and 40/10 FTTC connections (and 128kbit/s for 40/2). The document those are from may be a bit out of date now.
The interleaving depths indicate that the DLM has applied fairly high levels of interleaving (the maximum interleaving depth is 3072), but that doesn't explain the low upload speeds.

[#15]

I managed to get a PPPoE connection working on my laptop (with a wired connection). The speed test results still show 1.8mbps upload speed