From the looks of it, there is potentially nothing you can do.
But that would be the cowards way out, and perhaps I advise that there are some things you can try but do not anticipate a better result.
Since you are not experiencing errors to my knowledge, I doubt there is a fault at all. Perhaps your line is operating at the best it can. However, here are some things you can try if you wish:
- If you have a spare dangly filter (supplied with your modem-router combo) you could refer to https://kitz.co.uk/adsl/socket.htm on how to access your test socket. Usually, however, the impact between filters is typically at most a few hundred Kbps either way. Note: if you don't have a phone service, you do not need to filter it. But since you are using a Plusnet Hub One, I imagine you have a phone service provisioned.
- If you have any additional spiltters or devices plugged into your faceplate, this may interfere with broadband. Unplug anything unecessary from your socket except for the bare necessities for broadband (yes, even isolate your phone when you plug it all in to see if it makes a different the sync speed).
- If you have a different cable RJ11, you could try it. Though again, the difference is negligible unless you are using an excessively long one. It is best to keep the modem near the socket and use a shorter, high quality cable to absolutely maximise sync speed.
- I would advise replacing the Plusnet Hub One device. This is a personal recommendation since I have personally found it to be a detriment to performance and stability on even a short line. You are best looking for a modem that has a Broadcom chipset (there are a lot of people that can make recommendations on the site) compared to using a ISP supplied device that has a Lantiq chipset.
I may have missed a few things, and I imagine someone will fill the gaps. Though if you are not sure about something, feel free to ask.
Please bear in mind that if you are unplugging and plugging the modem back into the socket too often, it may cause the Dynamic Line Management to take negative actions against your line (since "retrains" over a certain threshold will trigger DLM action). Ensure you do not disconnect it too much in a day to prevent your line getting banded or raised SNR (which could decrease sync speed).