To answer your original question; a better modem may help, otherwise moan at your ISP.
About SNRM:
The SNRM is a measurement of the robustness vs speed tradeoff of the connection. It’s not a property of the line itself but is in your case chosen by BT’s DLM (dynamic line management) software which controls the settings in the two modems.
A higher SNRM means ‘more robust’ ie better error correction/error protection or less ambitious communication techniques, but slower speed. Higher SNRM is better, but you might prefer a lower SNRM for more unreliability and more speed.
A lower SNRM value means ‘fast, but less reliable’.
In your case your downstream SNRM will be either 6dB (slower, more reliable) or 3dB (faster, less reliable). I’m not sure what the story is regarding your upstream SNRM.
If your line is very short or very high quality or both, or interference is very low, or your line is ‘capped’ (ie speed limited) then you may get the max speed permitted and your SNRM may be at higher values than the usual 3dB/6dB downstream; not sure about upstream. This is a very good thing and you could go faster if only you were allowed to.
People who have G.INP have a second layer of powerful error correction/protection which allows you to have a lower SNRM and so go very fast without as much unreliability as normal. If you have G.INP and downstream 3dB SNRM this is a very high performance setting which should be ok for reliability for many people.
For most people, there will be a chosen ‘target SNRM’ set at the time of connection set-up, and the SNR will go up and down depending on the level of interference, throughout the day, usually being somewhat lower at night.
If the SNRM gets too low, then the modems will drop the connection and reconnect at lower speed, employing extra robustness.
People who have ADSL1/ADSL2/ADSL2+ can either change their target SNRM for both downstream and upstream themselves or ask their ISP to do it for them. People with FTTC/VDSL2 have no control over target SNRM, nor do their ISPs.
If you have G.FAST, which you don’t, then you will iirc have ‘SRA’ (seamless rate adaptation) ie. variable sync rate ie. variable speed and the modems will pick an SNRM and vary the speed according to the conditions. This means that you always get the maximum speed possible at all times. I’m slightly concerned about what this means for applications that measure initial speed and make decisions about service given the verdict. Mind you, effective speed could vary anyway because of variations in competing traffic or congestion, so my reservations must surely be illogical.